If you don't have any problems -- big or small -- while living in your apartment, that's awesome.  But you're probably in the minority.  When something goes wrong, contacting your landlord in writing, believe it or not, is the way to go.  Here's what you need to do and why it's important to keep a record of your correspondence:

The Chicago Residential Landlord and Tenant Ordinance (the CRLTO) sets forth, in surprisingly plain English, exactly what you should do in the event your landlord is not maintaining the premises in a fit and habitable condition (assuming the lease is governed by the municipal code) or in the event that your landlord is failing to make even minor repairs.  The first step, not surprisingly, is to deliver a written notice to the landlord specifying the exact problems with the apartment.  (This is merely a cursory summary of your rights and obligations.  If you should ever have a problem with the habitability of your dwelling unit, consult the CRLTO, which can be found online, or hire an attorney.)

Communicating in writing with your landlord is an excellent practice.  Even in cases where you are requesting simple repairs that don’t really impact the overall habitability of your apartment, you should correspond in writing and retain a copy of the email or letter.  If the problem is urgent, send a letter by certified mail, return receipt requested and send an e-mail too.

The CRLTO sets forth in great detail all the remedies available to aggrieved tenants, including the withholding of rent in certain circumstances.  Rent withholding can be a risky proposition, so consider seeking counsel (from a public legal services organization if need be) before taking any such action.  And for more insight on these issues, see this post from the the Metropolitan Tenants Organization.