In Illinois, an individual can legally break a lease for one of the following reasons: the premise is unsafe, the tenant or a child has been sexually assaulted, the landlord violated privacy rights, or the tenant is going into the military. It's difficult to legally break a lease for other reasons.
Even if a tenant does not have a legal justification to break his lease, he may still not be held liable for the remaining rent under the terms of the lease. The law mandates that landlord's must make a good-faith effort to re-rent the unit before they impose a bill for the remaining rent on the tenant. When this happens, the tenant may only be liable to pay for the rent due between the time he moved out to the time the landlord found a new tenant to take over.
If a tenant breaks his lease, the landlord can't just sit back and wait until the end of that lease before attempting to find another renter. The landlord does not have to rent the unit for anything less than the current fair market value, and if the landlord is unable to re-rent the unit, the tenant is held liable for the rent under the remainder of his lease term.