The Republican presidential candidates -- particularly Mitt Romney -- have been under pressure to release their tax returns. The topic has gotten plenty of play in the GOP debates this year. It's not the first time the issue has come up in a presidential campaign, however. Although tax returns are private, most presidents since the 1970s have chosen to release them.
In honor of Presidents Day, here's a look back at how much some past presidents made, and how they earned it. All numbers come from tax returns courtesy of the Tax History Project.