diff --git a/CHANGELOG.md b/CHANGELOG.md
index bdb535a9..b2964a2f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,7 @@ and this project does not currently adhere to a particular versioning scheme.
 - Add `log` and `atan2` builtin functions. ([#583][gh-583])
 - Add `to_f24`, `to_u24` and `to_i24` number casting builtin functions. ([#582][gh-582])
 - Add `IO/sleep` builtin function to sleep for a given amount of seconds as a float. ([#581][gh-581])
+- Add primitive file IO functions `IO/FS/{read, write, seek, open, close}`. ([#573][gh-573])
 
 ## [0.2.35] - 2024-06-06
 
@@ -344,6 +345,7 @@ and this project does not currently adhere to a particular versioning scheme.
 [gh-526]: https://github.com/HigherOrderCO/Bend/issues/526
 [gh-528]: https://github.com/HigherOrderCO/Bend/issues/528
 [gh-581]: https://github.com/HigherOrderCO/Bend/issues/581
+[gh-573]: https://github.com/HigherOrderCO/Bend/issues/573
 [gh-582]: https://github.com/HigherOrderCO/Bend/issues/582
 [gh-583]: https://github.com/HigherOrderCO/Bend/issues/583
 [gh-586]: https://github.com/HigherOrderCO/Bend/issues/586
diff --git a/docs/builtins.md b/docs/builtins.md
index d20dca4d..0f9fa60e 100644
--- a/docs/builtins.md
+++ b/docs/builtins.md
@@ -225,7 +225,74 @@ A Natural Number can be written with literals with a `#` before the literal numb
 
 
 ## IO
-IO Functions are in the **next milestone**!
+The basic builtin IO functions are under development and will be stable in the next milestone.
+
+Here is the current list of functions, but be aware that they may change in the near future.
+
+### File IO
+
+#### File open
+```python
+def IO/FS/open(path, mode)
+```
+
+Opens a file with with `path` being given as a string and `mode` being a string with the mode to open the file in. The mode should be one of the following:
+- `"r"`: Read mode
+- `"w"`: Write mode (write at the beginning of the file, overwriting any existing content)
+- `"a"`: Append mode (write at the end of the file)
+- `"r+"`: Read and write mode
+- `"w+"`: Read and write mode
+- `"a+"`: Read and append mode
+
+Returns an U24 with the file descriptor. File descriptors are not necessarily the same as the ones assigned by the operating system, but rather unique identifiers internal to Bend's runtime.
+
+#### File descriptors for standard files
+
+The standard input/output files are always open and assigned the following file descriptors:
+- `IO/FS/STDIN = 0`: Standard input
+- `IO/FS/STDOUT = 1`: Standard output
+- `IO/FS/STDERR = 2`: Standard error
+
+#### File close
+```python
+def IO/FS/close(file)
+```
+
+Closes the file with the given `file` descriptor.
+
+#### File read
+```python
+def IO/FS/read(file, num_bytes)
+```
+
+Reads `num_bytes` bytes from the file with the given `file` descriptor.
+
+Returns a list of U24 with each element representing a byte read from the file.
+
+#### File write
+```python
+def IO/FS/write(file, bytes)
+```
+
+Writes `bytes`, a list of U24 with each element representing a byte, to the file with the given `file` descriptor.
+
+Returns nothing (`*`).
+
+Writing discards any preexisting content that came after the current position. For example, if your file contains the text `Hello, world!` and the current position is at the `,`, writing `!` will 
+
+#### File seek
+```python
+def IO/FS/seek(file, offset, mode)
+```
+
+Moves the current position of the file with the given `file` descriptor to the given `offset`, an I24 or U24 number, in bytes.
+
+`mode` can be one of the following:
+- `IO/FS/SEEK_SET = 0`: Seek from start of file
+- `IO/FS/SEEK_CUR = 1`: Seek from current position
+- `IO/FS/SEEK_END = 2`: Seek from end of file
+
+Returns nothing (`*`).
 
 ## Numeric operations
 
diff --git a/src/fun/builtins.bend b/src/fun/builtins.bend
index 6c8fe2cc..ab072085 100644
--- a/src/fun/builtins.bend
+++ b/src/fun/builtins.bend
@@ -88,13 +88,7 @@ def IO/bind(a, b):
 def call(func, argm):
   return IO/Call(IO/MAGIC, func, argm, lambda x: IO/Done(IO/MAGIC, x))
 
-
-print text = (IO/Call IO/MAGIC "WRITE" (1, text) @x (IO/Done IO/MAGIC x))
-#input = (IO/Call IO/MAGIC "GET_TEXT" * @x (IO/Done IO/MAGIC x))
-
-#read_file path = (IO/Call IO/MAGIC "GET_FILE" path @x (IO/Done IO/MAGIC x))
-#write_file path text = (IO/Call IO/MAGIC "PUT_FILE" (path, text) @x (IO/Done IO/MAGIC x))
-
+## Time and sleep
 # Returns a monotonically increasing nanosecond timestamp as an u48 encoded as a pair of u24s.
 IO/get_time = (IO/Call IO/MAGIC "GET_TIME" * @x (IO/Done IO/MAGIC x))
 
@@ -108,6 +102,40 @@ def IO/sleep(seconds):
   hi = to_u24(nanos / 0x1_000_000.0)
   return IO/nanosleep((hi, lo))
 
+## File IO
+
+### File IO primitives
+IO/FS/open path mode = (IO/Call IO/MAGIC "OPEN" (path, mode) @x (IO/Done IO/MAGIC x))
+IO/FS/close file = (IO/Call IO/MAGIC "CLOSE" file @x (IO/Done IO/MAGIC x))
+IO/FS/read file num_bytes = (IO/Call IO/MAGIC "READ" (file, num_bytes) @x (IO/Done IO/MAGIC x))
+IO/FS/write file bytes = (IO/Call IO/MAGIC "WRITE" (file, bytes) @x (IO/Done IO/MAGIC x))
+IO/FS/seek file offset mode = (IO/Call IO/MAGIC "SEEK" (file, (offset, mode)) @x (IO/Done IO/MAGIC x))
+
+### Always available files
+IO/FS/STDIN = 0
+IO/FS/STDOUT = 1
+IO/FS/STDERR = 2
+
+### Seek modes
+# Seek from start of file
+IO/FS/SEEK_SET = +0
+# Seek from current position
+IO/FS/SEEK_CUR = +1
+# Seek from end of file
+IO/FS/SEEK_END = +2
+
+### File utilities
+#read_file path = (IO/Call IO/MAGIC "GET_FILE" path @x (IO/Done IO/MAGIC x))
+#write_file path text = (IO/Call IO/MAGIC "PUT_FILE" (path, text) @x (IO/Done IO/MAGIC x))
+
+### Standard input and output utilities
+
+# TODO: Encode with utf-8
+IO/print text = (IO/FS/write IO/FS/STDOUT text)
+
+# TODO: Should read any length of input
+IO/input = (IO/FS/read IO/FS/STDIN 1024)
+
 # Lazy thunks
 # We can defer the evaluation of a function by wrapping it in a thunk
 # Ex: @x (x @arg1 @arg2 @arg3 (f arg1 arg2 arg3) arg1 arg2 arg3)