An Illinois roommate agreement is a document executed by roommates to determine the responsibilities and regulations that all parties to the agreement must follow while sharing a residence. The contract does not involve the landlord, and it’s possible that not all roommates are signatories on the original lease document. Once completed, the document will relay the portion of rent, utility bills, and other expenses that each roommate will be responsible for paying. It is also recommended to include a set of rules that determine cleaning duties and prohibited behavior (excessive noise, unauthorized guests, smoking, alcohol/drug use, keeping pets, etc.).