ONLINE_HELP, BOOK='*.pdf" works fine for my team. However, a person testing under Windows NT reported a crash problem. I tried to put some CATCH errors around the online_help call and tried to call with an invalid file name. Turns out that nothing gets reported back. Also, when at the IDLDE command line and calling online_help with a wrong .pdf file name nothing happens; no error gets reported back at all, and the pdf viewer does not show. Is it possible to catch for these online_help errors? Any ideas of why this command would not work under Windows NT? We are developing via IDL 6.2 on a Linux box.