The IDL Astronomy User’s Library. IDL (Interactive Data Language) is a commercial plotting, image processing, and programming language that is widely used in astronomy. Although IDL is a powerful language for spectral and image processing, it does not contain applications specific to astronomy. Therefore, in 1990 I created the IDL Astronomy User’s Library (AUL), which is a collection of astronomy-related procedures written in the IDL language, available via anonymous FTP from idlastro.gsfc.nasa.gov. I summarize recent developments in the IDL language of particular interest to astronomers. I then mention some additions to the IDL Astronomy Library since the previous report of Landsman (1993). Finally, I critically examine possible drawbacks to the use of IDL for astronomical data analysis.