A tenancy at will is terminable by either party or upon the death of either party. No particular formal notice to terminate is required. However, some courts have required the landowner to demand possession and allow a reasonable time for the occupant to vacate.
If a lease is terminable at the will of only one party courts have interpreted the agreement variously as terminable by either party, a life estate, or even a fee simple title.
A tenancy at will may be created by agreement. There is a preference at common law for finding a periodic tenancy, but a tenancy at will is created by implication when the parties agree to possession but set no term or rental period or otherwise agree to terms from which an intended duration can be determined. Also, while in most states a lease that does not conform to the statute of frauds results in a periodic tenancy in some states by statute such a situation results in a tenancy at will.
Legislation in several jurisdictions affect the common law rules concerning tenancies at will. In some, the tenancy at will can only be created by a written instrument while in others any tenancy not created by a written instrument is a tenancy at will.
Also, in some states statutes require formal written notice to terminate a tenancy at will, while others have done away with notice requirements for such tenancies altogether. In states requiring formal notice it is not always clear if this is the only way to terminate such a tenancy, or whether the common law rule that such a tenancy terminates upon death of either party still operates.
Robert S. Schonshinski, American Law of Landlord and Tenant, §§2:16-2:19 (1980) and Cumulative Supplement (2008).