# External Development Guide

IDL provides ways to link to code written in other programming languages. See the available External Linking IDL routines and functions.

You can find more in-depth information in the External Development Guide found in the `help\pdf` directory of your IDL installation. This book covers topics such as:

- Using SPAWN and Pipes
- Using CALL_EXTERNAL
- Remote Procedure Calls
- IDL Types
- Handling Keywords, Variable, File I/O, Timers, and Errors
- Adding System Routines
- Callable IDL
- Adding External Widgets to IDL