Chicago holds city elections in February 2015. There are three citywide offices, Mayor, Treasurer, and City Clerk. The elections are non-partisan. All candidates need 12,500 signatures to get on the ballot. If no one gets 50% in February, there is a run-off in April.
For Mayor, ten candidates submitted a petition, but one candidate then withdrew, leaving nine petitions. For Treasurer, only one person filed. For City Clerk, two candidates filed. From those remaining twelve petitions, nine have been challenged, including both petitions for City Clerk. Here is a link to the Chicago Board of Elections web page listing the petitions and their status.
The challenges will be heard starting on Friday, December 19. Here is an article about Willie Wilson, one of the Mayoral candidates whose petition was challenged, even though he submitted 47,500 signatures. Here is a more recent article about Wilson and his reaction to the challenge of his petition.