To make it on the ballot, candidates need to submit thousands of signatures from members of his or her party.
Campaigns have until Wednesday to do it. But Democratic front-runner Hillary Clinton - as well as Republicans Jeb Bush, Chris Christie, Ted Cruz, Marco Rubio and Donald Trump - each had theirs in bright and early. A press release says supporters for Bernie Sanders, who's seeking the Democratic nomination, will submit his Wednesday.
These presidential filings, while necessary, are only part of the story.
Under Republican Party rules, candidates need to concentrate on getting local supporters to file, then win elections, as convention delegates. In Illinois, it's this contest for delegates - not the popular vote - that matters.
Democrats in Illinois need delegates too, but the party's hybrid system also factors in the true top-of-the-ticket results.
Illinois' primary election is in mid-March.