Charles,

>What is required in terms of reference material to learn how to have an app determine if an instance of itself was already running when the user tries to open one of it's main forms, then second instance make the original instance open the form, and then quit.

Why not just keep it simple and prohibit starting the app if one instance is already running. Don't put "Clients" or "Invoice" as separate options on the Windows Start menu, if that's what you're doing.

Just have an option to start the main app, then put up a little form or toolbar that can stay up and accessible to let the user choose which forms to open.

Otherwise you're looking at having an app-level timer kicking off constantly in your first app checking some kind of message queue or log written to by your second VFP instance before it shuts down. Sounds messy.