"Headers already sent" error message

I'm having a bit of trouble understanding why I'm getting a "Cannot add header information; Headers already sent" error message. I have several pages where I'm using the headers with no problems...but this single page I keep getting the error. The only real difference is that this page has a bunch of logic checks before it calls the header command. What is the cause of this error? what sort of things should I check for? I'm lost, Ron