If I have a form and want certain fields to be invisible in the event certain fields meet certain criteria, where do I place the criteria?

It depends on the actual events that you want to respond to. To respond to changes in the value of a particular field, put the code (or the call to a Sub) in that field's AfterUpdate event procedure. To respond to the state of each record as it is displayed, use the form's OnCurrent event. If you're dealing with multiple events and/or fields, I'd set up one Sub to do the entire job, and simply call it from multiple locations.