diff --git a/AK/FileSystemPath.h b/AK/FileSystemPath.h
index 4cf257dbdc6..9400292c67d 100644
--- a/AK/FileSystemPath.h
+++ b/AK/FileSystemPath.h
@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 
 namespace AK {
 
diff --git a/AK/Forward.h b/AK/Forward.h
new file mode 100644
index 00000000000..e1b66768f1c
--- /dev/null
+++ b/AK/Forward.h
@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+namespace AK {
+
+class Bitmap;
+class ByteBuffer;
+class IPv4Address;
+class JsonArray;
+class JsonObject;
+class JsonValue;
+class String;
+class StringBuilder;
+class StringImpl;
+class StringView;
+class URL;
+
+template<typename T>
+class SinglyLinkedList;
+
+template<typename T>
+class DoublyLinkedList;
+
+template<typename T>
+class InlineLinkedList;
+
+template<typename T, int capacity>
+class CircularQueue;
+
+template<typename T>
+class Badge;
+
+template<typename T>
+class FixedArray;
+
+template<typename>
+class Function;
+
+template<typename Out, typename... In>
+class Function<Out(In...)>;
+
+template<typename T>
+class NonnullRefPtr;
+
+template<typename T>
+class NonnullOwnPtr;
+
+template<typename T>
+class RefPtr;
+
+template<typename T>
+class OwnPtr;
+
+template<typename T>
+class WeakPtr;
+
+template<typename T, int inline_capacity = 0>
+class Vector;
+
+}
+
+using AK::Badge;
+using AK::Bitmap;
+using AK::ByteBuffer;
+using AK::CircularQueue;
+using AK::DoublyLinkedList;
+using AK::FixedArray;
+using AK::Function;
+using AK::InlineLinkedList;
+using AK::IPv4Address;
+using AK::JsonArray;
+using AK::JsonObject;
+using AK::JsonValue;
+using AK::NonnullOwnPtr;
+using AK::NonnullRefPtr;
+using AK::OwnPtr;
+using AK::RefPtr;
+using AK::SinglyLinkedList;
+using AK::String;
+using AK::StringBuilder;
+using AK::StringImpl;
+using AK::StringView;
+using AK::URL;
+using AK::Vector;
diff --git a/AK/IPv4Address.h b/AK/IPv4Address.h
index be729938e23..6d5dc6f5353 100644
--- a/AK/IPv4Address.h
+++ b/AK/IPv4Address.h
@@ -26,10 +26,11 @@
 
 #pragma once
 
-#include <AK/String.h>
 #include <AK/LogStream.h>
 #include <AK/NetworkOrdered.h>
 #include <AK/Optional.h>
+#include <AK/String.h>
+#include <AK/Vector.h>
 
 typedef u32 in_addr_t;
 
diff --git a/AK/JsonValue.h b/AK/JsonValue.h
index a109b671bee..c0780e331c7 100644
--- a/AK/JsonValue.h
+++ b/AK/JsonValue.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/Forward.h>
 #include <AK/IPv4Address.h>
 #include <AK/Optional.h>
 #include <AK/String.h>
@@ -33,10 +34,6 @@
 
 namespace AK {
 
-class JsonArray;
-class JsonObject;
-class StringBuilder;
-
 class JsonValue {
 public:
     enum class Type {
diff --git a/AK/LogStream.cpp b/AK/LogStream.cpp
index b6d2b5396d8..c6702fea6cb 100644
--- a/AK/LogStream.cpp
+++ b/AK/LogStream.cpp
@@ -112,4 +112,10 @@ DebugLogStream dbg()
     return stream;
 }
 
+DebugLogStream::~DebugLogStream()
+{
+    char newline = '\n';
+    write(&newline, 1);
+}
+
 }
diff --git a/AK/LogStream.h b/AK/LogStream.h
index 6cf7b45ddd2..8e00ad278df 100644
--- a/AK/LogStream.h
+++ b/AK/LogStream.h
@@ -62,11 +62,7 @@ private:
 class DebugLogStream final : public LogStream {
 public:
     DebugLogStream() {}
-    virtual ~DebugLogStream() override
-    {
-        char newline = '\n';
-        write(&newline, 1);
-    }
+    virtual ~DebugLogStream() override;
 
     virtual void write(const char* characters, int length) const override
     {
diff --git a/AK/String.cpp b/AK/String.cpp
index 4e064b8070d..4149a15467e 100644
--- a/AK/String.cpp
+++ b/AK/String.cpp
@@ -27,10 +27,10 @@
 #include <AK/StdLibExtras.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
-#include <stdarg.h>
+#include <AK/Vector.h>
 
 #ifndef KERNEL
-#include <inttypes.h>
+#    include <inttypes.h>
 #endif
 
 #ifdef KERNEL
diff --git a/AK/String.h b/AK/String.h
index 71c4a48293c..fea82b40b00 100644
--- a/AK/String.h
+++ b/AK/String.h
@@ -26,12 +26,11 @@
 
 #pragma once
 
-#include <AK/ByteBuffer.h>
+#include <AK/Forward.h>
 #include <AK/RefPtr.h>
 #include <AK/StringImpl.h>
 #include <AK/StringView.h>
 #include <AK/Traits.h>
-#include <AK/Vector.h>
 
 namespace AK {
 
diff --git a/AK/StringBuilder.cpp b/AK/StringBuilder.cpp
index e60f1c88267..da97a970c86 100644
--- a/AK/StringBuilder.cpp
+++ b/AK/StringBuilder.cpp
@@ -27,7 +27,7 @@
 #include <AK/PrintfImplementation.h>
 #include <AK/StdLibExtras.h>
 #include <AK/StringBuilder.h>
-#include <stdarg.h>
+#include <AK/String.h>
 
 namespace AK {
 
@@ -96,6 +96,11 @@ String StringBuilder::to_string()
     return String((const char*)m_buffer.data(), m_length);
 }
 
+String StringBuilder::build()
+{
+    return to_string();
+}
+
 StringView StringBuilder::string_view() const
 {
     return StringView { (const char*)m_buffer.data(), m_length };
diff --git a/AK/StringBuilder.h b/AK/StringBuilder.h
index de99291bf50..1ffa801ddd3 100644
--- a/AK/StringBuilder.h
+++ b/AK/StringBuilder.h
@@ -26,8 +26,8 @@
 
 #pragma once
 
-#include <AK/String.h>
-#include <AK/Vector.h>
+#include <AK/ByteBuffer.h>
+#include <AK/Forward.h>
 #include <stdarg.h>
 
 namespace AK {
@@ -45,8 +45,7 @@ public:
     void appendf(const char*, ...);
     void appendvf(const char*, va_list);
 
-    String build() { return to_string(); }
-
+    String build();
     String to_string();
     ByteBuffer to_byte_buffer();
 
diff --git a/AK/StringView.cpp b/AK/StringView.cpp
index 7082b608d74..69657e78cde 100644
--- a/AK/StringView.cpp
+++ b/AK/StringView.cpp
@@ -24,8 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/ByteBuffer.h>
 #include <AK/String.h>
 #include <AK/StringView.h>
+#include <AK/Vector.h>
 
 namespace AK {
 
diff --git a/AK/StringView.h b/AK/StringView.h
index 6eab64ceec4..4bf12a53644 100644
--- a/AK/StringView.h
+++ b/AK/StringView.h
@@ -26,14 +26,11 @@
 
 #pragma once
 
-#include <AK/Vector.h>
+#include <AK/Forward.h>
+#include <AK/StdLibExtras.h>
 
 namespace AK {
 
-class ByteBuffer;
-class String;
-class StringImpl;
-
 class StringView {
 public:
     StringView() {}
diff --git a/AK/Utf8View.cpp b/AK/Utf8View.cpp
index 847bef01f74..885721f7e98 100644
--- a/AK/Utf8View.cpp
+++ b/AK/Utf8View.cpp
@@ -24,8 +24,9 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <AK/Utf8View.h>
+#include <AK/Assertions.h>
 #include <AK/LogStream.h>
+#include <AK/Utf8View.h>
 
 namespace AK {
 
diff --git a/AK/Vector.h b/AK/Vector.h
index ed8b02b576c..f8d003e6cd0 100644
--- a/AK/Vector.h
+++ b/AK/Vector.h
@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/Forward.h>
 #include <AK/StdLibExtras.h>
 #include <AK/Traits.h>
 #include <AK/kmalloc.h>
@@ -44,9 +45,6 @@
 
 namespace AK {
 
-template<typename T, int inline_capacity>
-class Vector;
-
 template<typename VectorType, typename ElementType>
 class VectorIterator {
 public:
@@ -132,7 +130,7 @@ public:
     }
 };
 
-template<typename T, int inline_capacity = 0>
+template<typename T, int inline_capacity>
 class Vector {
 public:
     Vector()
diff --git a/Applications/HexEditor/HexEditor.h b/Applications/HexEditor/HexEditor.h
index 42adcb105e8..9e1ce1f7136 100644
--- a/Applications/HexEditor/HexEditor.h
+++ b/Applications/HexEditor/HexEditor.h
@@ -26,13 +26,14 @@
 
 #pragma once
 
+#include <AK/ByteBuffer.h>
 #include <AK/Function.h>
 #include <AK/HashMap.h>
 #include <AK/NonnullOwnPtrVector.h>
 #include <AK/NonnullRefPtrVector.h>
 #include <AK/StdLibExtras.h>
-#include <LibGfx/TextAlignment.h>
 #include <LibGUI/ScrollableWidget.h>
+#include <LibGfx/TextAlignment.h>
 
 class HexEditor : public GUI::ScrollableWidget {
     C_OBJECT(HexEditor)
diff --git a/Libraries/LibC/grp.cpp b/Libraries/LibC/grp.cpp
index 21d4a736501..4b0726875d0 100644
--- a/Libraries/LibC/grp.cpp
+++ b/Libraries/LibC/grp.cpp
@@ -25,6 +25,7 @@
  */
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 #include <grp.h>
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/Libraries/LibC/pwd.cpp b/Libraries/LibC/pwd.cpp
index 794438a51f5..00bf40476e7 100644
--- a/Libraries/LibC/pwd.cpp
+++ b/Libraries/LibC/pwd.cpp
@@ -25,6 +25,7 @@
  */
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 #include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/Libraries/LibC/syslog.cpp b/Libraries/LibC/syslog.cpp
index 180eb22fdeb..650347da967 100644
--- a/Libraries/LibC/syslog.cpp
+++ b/Libraries/LibC/syslog.cpp
@@ -27,6 +27,7 @@
 // Has to be defined before including due to legacy Unices
 #define SYSLOG_NAMES 1
 
+#include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <stdio.h>
 #include <string.h>
diff --git a/Libraries/LibELF/ELFDynamicLoader.h b/Libraries/LibELF/ELFDynamicLoader.h
index e5cc508e892..c3932714d48 100644
--- a/Libraries/LibELF/ELFDynamicLoader.h
+++ b/Libraries/LibELF/ELFDynamicLoader.h
@@ -26,14 +26,14 @@
 
 #pragma once
 
-#include <LibELF/ELFDynamicObject.h>
-#include <LibELF/ELFImage.h>
-#include <LibELF/exec_elf.h>
-#include <mman.h>
-
+#include <AK/Assertions.h>
 #include <AK/OwnPtr.h>
 #include <AK/RefCounted.h>
 #include <AK/String.h>
+#include <LibELF/ELFDynamicObject.h>
+#include <LibELF/ELFImage.h>
+#include <LibELF/exec_elf.h>
+#include <sys/mman.h>
 
 #define ALIGN_ROUND_UP(x, align) ((((size_t)(x)) + align - 1) & (~(align - 1)))
 
diff --git a/Libraries/LibELF/ELFDynamicObject.cpp b/Libraries/LibELF/ELFDynamicObject.cpp
index 0e8aafaf916..5a98a0ae043 100644
--- a/Libraries/LibELF/ELFDynamicObject.cpp
+++ b/Libraries/LibELF/ELFDynamicObject.cpp
@@ -24,12 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
 #include <LibELF/ELFDynamicObject.h>
 #include <LibELF/exec_elf.h>
-
-#include <AK/StringBuilder.h>
-
-#include <assert.h>
 #include <stdio.h>
 
 static const char* name_for_dtag(Elf32_Sword d_tag);
diff --git a/Libraries/LibELF/ELFDynamicObject.h b/Libraries/LibELF/ELFDynamicObject.h
index 5e88f2665d6..80eb9db281b 100644
--- a/Libraries/LibELF/ELFDynamicObject.h
+++ b/Libraries/LibELF/ELFDynamicObject.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/Assertions.h>
 #include <LibBareMetal/Memory/VirtualAddress.h>
 #include <LibELF/exec_elf.h>
 
diff --git a/Libraries/LibGUI/ProgressBar.cpp b/Libraries/LibGUI/ProgressBar.cpp
index 4e55e3720b2..b957a159c1b 100644
--- a/Libraries/LibGUI/ProgressBar.cpp
+++ b/Libraries/LibGUI/ProgressBar.cpp
@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/Assertions.h>
 #include <AK/StringBuilder.h>
 #include <LibGUI/Painter.h>
 #include <LibGUI/ProgressBar.h>
diff --git a/Libraries/LibGUI/ScrollBar.cpp b/Libraries/LibGUI/ScrollBar.cpp
index c50f3fb7b40..27c3531ad96 100644
--- a/Libraries/LibGUI/ScrollBar.cpp
+++ b/Libraries/LibGUI/ScrollBar.cpp
@@ -24,12 +24,14 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <LibGfx/CharacterBitmap.h>
-#include <LibGfx/Bitmap.h>
-#include <LibGfx/Palette.h>
-#include <LibGfx/StylePainter.h>
+#include <AK/Assertions.h>
+#include <AK/StdLibExtras.h>
 #include <LibGUI/Painter.h>
 #include <LibGUI/ScrollBar.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/CharacterBitmap.h>
+#include <LibGfx/Palette.h>
+#include <LibGfx/StylePainter.h>
 
 namespace GUI {
 
diff --git a/Libraries/LibGUI/Shortcut.cpp b/Libraries/LibGUI/Shortcut.cpp
index 37fa479450a..cc76d78a759 100644
--- a/Libraries/LibGUI/Shortcut.cpp
+++ b/Libraries/LibGUI/Shortcut.cpp
@@ -25,6 +25,7 @@
  */
 
 #include <AK/StringBuilder.h>
+#include <AK/Vector.h>
 #include <LibGUI/Shortcut.h>
 
 namespace GUI {
diff --git a/Libraries/LibGfx/PNGLoader.cpp b/Libraries/LibGfx/PNGLoader.cpp
index 0b293dbac7d..863c63d8067 100644
--- a/Libraries/LibGfx/PNGLoader.cpp
+++ b/Libraries/LibGfx/PNGLoader.cpp
@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/ByteBuffer.h>
 #include <AK/FileSystemPath.h>
 #include <AK/MappedFile.h>
 #include <AK/NetworkOrdered.h>
diff --git a/Libraries/LibGfx/Painter.h b/Libraries/LibGfx/Painter.h
index 7e8cfb838f2..1775c1b3d2f 100644
--- a/Libraries/LibGfx/Painter.h
+++ b/Libraries/LibGfx/Painter.h
@@ -26,12 +26,13 @@
 
 #pragma once
 
-#include "Color.h"
-#include "Point.h"
-#include "Rect.h"
-#include "Size.h"
 #include <AK/String.h>
 #include <AK/Utf8View.h>
+#include <AK/Vector.h>
+#include <LibGfx/Color.h>
+#include <LibGfx/Point.h>
+#include <LibGfx/Rect.h>
+#include <LibGfx/Size.h>
 #include <LibGfx/TextAlignment.h>
 #include <LibGfx/TextElision.h>
 
diff --git a/Libraries/LibGfx/Rect.cpp b/Libraries/LibGfx/Rect.cpp
index bff5d90ae31..af663bfc092 100644
--- a/Libraries/LibGfx/Rect.cpp
+++ b/Libraries/LibGfx/Rect.cpp
@@ -24,8 +24,9 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include "Rect.h"
 #include <AK/StdLibExtras.h>
+#include <AK/Vector.h>
+#include <LibGfx/Rect.h>
 
 namespace Gfx {
 
diff --git a/Libraries/LibHTML/CSS/StyleDeclaration.h b/Libraries/LibHTML/CSS/StyleDeclaration.h
index 52a0e8ddc7b..ebe8a6d3ca8 100644
--- a/Libraries/LibHTML/CSS/StyleDeclaration.h
+++ b/Libraries/LibHTML/CSS/StyleDeclaration.h
@@ -27,6 +27,7 @@
 #pragma once
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 #include <LibHTML/CSS/StyleValue.h>
 
 struct StyleProperty {
diff --git a/Libraries/LibHTML/CSS/StyleValue.cpp b/Libraries/LibHTML/CSS/StyleValue.cpp
index 591e8bdcaa3..0a19807c811 100644
--- a/Libraries/LibHTML/CSS/StyleValue.cpp
+++ b/Libraries/LibHTML/CSS/StyleValue.cpp
@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/ByteBuffer.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/PNGLoader.h>
 #include <LibHTML/CSS/StyleValue.h>
diff --git a/Libraries/LibHTML/DOM/HTMLImageElement.h b/Libraries/LibHTML/DOM/HTMLImageElement.h
index 7035b1706d5..345c32665c3 100644
--- a/Libraries/LibHTML/DOM/HTMLImageElement.h
+++ b/Libraries/LibHTML/DOM/HTMLImageElement.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/ByteBuffer.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/ImageDecoder.h>
 #include <LibHTML/DOM/HTMLElement.h>
diff --git a/Libraries/LibIPC/Message.h b/Libraries/LibIPC/Message.h
index a8f4bccd5b2..73dc9c689d0 100644
--- a/Libraries/LibIPC/Message.h
+++ b/Libraries/LibIPC/Message.h
@@ -26,7 +26,7 @@
 
 #pragma once
 
-#include <AK/String.h>
+#include <AK/Vector.h>
 
 namespace IPC {
 
diff --git a/Libraries/LibMarkdown/MDBlock.h b/Libraries/LibMarkdown/MDBlock.h
index b22265f4228..facceb359ae 100644
--- a/Libraries/LibMarkdown/MDBlock.h
+++ b/Libraries/LibMarkdown/MDBlock.h
@@ -26,7 +26,7 @@
 
 #pragma once
 
-#include <AK/String.h>
+#include <AK/Vector.h>
 
 class MDBlock {
 public:
diff --git a/Servers/LookupServer/DNSRequest.h b/Servers/LookupServer/DNSRequest.h
index ae42a0fe35e..2b55dd75fbc 100644
--- a/Servers/LookupServer/DNSRequest.h
+++ b/Servers/LookupServer/DNSRequest.h
@@ -27,9 +27,8 @@
 #pragma once
 
 #include "DNSQuestion.h"
-#include <AK/ByteBuffer.h>
-#include <AK/String.h>
 #include <AK/Types.h>
+#include <AK/Vector.h>
 
 #define T_A 1
 #define T_NS 2
diff --git a/Servers/ProtocolServer/Download.h b/Servers/ProtocolServer/Download.h
index 360e407c476..4653d69eab5 100644
--- a/Servers/ProtocolServer/Download.h
+++ b/Servers/ProtocolServer/Download.h
@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/ByteBuffer.h>
 #include <AK/RefCounted.h>
 #include <AK/URL.h>
 #include <AK/WeakPtr.h>
diff --git a/Userland/chown.cpp b/Userland/chown.cpp
index d5009dff244..54202c2f72a 100644
--- a/Userland/chown.cpp
+++ b/Userland/chown.cpp
@@ -25,6 +25,7 @@
  */
 
 #include <AK/String.h>
+#include <AK/Vector.h>
 #include <grp.h>
 #include <pwd.h>
 #include <stdio.h>
@@ -69,7 +70,7 @@ int main(int argc, char** argv)
         if (!ok) {
             new_gid = getgrnam(parts[1].characters())->gr_gid;
 
-            if(!new_gid) {
+            if (!new_gid) {
                 fprintf(stderr, "Invalid gid: '%s'\n", parts[1].characters());
                 return 1;
             }