Lease-breaking tenants may have a defense to a landlord's suit for unpaid rent–rent the landlord would have collected had the tenant stayed in the rental. Landlords file these lawsuits when there's considerable time left on the lease, and their reasonable efforts, if legally required (see the section above), have failed to produce a new tenant (perhaps because the market is soft, or the rental just isn't that attractive). As explained above, landlords usually don't bother filing suit unless they can find the tenant, have some reason to think that they can actually collect if they win, and are confident that the tenant has no legal defense for failing to stay through the end of the lease.
The most common defense to breaking a lease is the tenant's claim that the rental was uninhabitable under state law. Landlords must offer and maintain "fit and habitable" premises, and if they fail to do so, their failure justifies the tenant's lease-breaking. To successfully raise this defense, the tenants must prove that they did not cause the problem themselves, the defects were serious enough to threaten health or safety or constituted a lack of basic services, and the landlord had been given a reasonable amount of time to fix the problem.
When two or more people rent property together and all sign the same rental agreement or lease, they are cotenants. Each cotenant shares the same rights and responsibilities under the lease or rental agreement. For example, all cotenants, regardless of agreements they make among themselves, are liable to the landlord for the entire amount of the rent. But being cotenants together does not establish one cotenant's legal right to sue the other cotenant for his share of the rent. In order to do that, the cotenants need to have an agreement between them that specifies how much each will contribute to the total rent. A cotenant who doesn't pay his share, as specified in the agreement, may be sued by the other cotenant(s) in small claims court.
EXAMPLE: James and Helen sign a month-to-month rental agreement for an $800 apartment. They agree between themselves to each pay half of the rent. After three months, James moves out without notifying Helen or the owner, Laura. As one of the two cotenants, Helen is still legally obligated to pay all the rent and may be able to recover James's share by suing him in small claims court. In the same way, Laura, the owner, may serve Helen, the tenant, with a notice to pay the whole rent or leave (or face eviction).
In the cities in New York and California that have rent control laws, a landlord who charges illegally high rent can be sued by the tenant not only for the excess rent charged, but for a punitive amount as well (often several times the overcharge).