Whilst other tenancy agreements exist, most people renting have an Assured Shorthold Tenancy Agreement which usually lasts for a minimum of 6 months. You can agree to a set term e.g. 6 or 12 months, or you can leave the term open ended. Once the initial 6 month period ends you have the option of renewing for another fixed term or allowing the tenancy to run-on on a periodic basis, this being the rent period - monthly or weekly. The tenancy then becomes a Periodic Tenancy.