Broadly speaking, the lease will be enforceable against the party or parties who signed the lease, even if others did not. A common example is where tenants sign a lease and move into an apartment, but the landlord doesn't also sign. The tenants are bound to the lease even without the landlord's signature.

The lease itself may complicate the analysis. For example, sometimes a lease will provide that it is not valid unless it is signed by all named tenants, so that if only one tenant has signed the landlord may deny occupancy. It may also provide that it is not valid unless countersigned by the landlord, meaning that even after the tenants sign the landlord may be able to reject the lease. The landlord cannot bind the tenants by signing the lease, if no tenant has signed.

If there is a problem with the execution of a lease but the tenant is allowed to take occupancy, the partial performance of the contract may allow for enforcement of the lease even if the party who would prefer to avoid the lease hasn't signed. That is, a landlord can't try to convert an annual lease to a month-to-month tenancy by protesting that he didn't sign the lease after allowing the tenants to move in. However, a co-tenant who does not sign a lease could potentially be shielded from claims for rent or damage in the event of an eviction or a problem with the rental unit at the end of the occupancy.