The rent index is a central instrument of German tenancy law, serving as a reference guide for landlords, tenants, and courts. It is typically updated every two years and captures rents actually paid over the past four to six years. A distinction is made between the simple rent index, which provides a general overview, and the qualified rent index, which is compiled according to scientific principles and carries stronger legal weight. Since 2022, municipalities above a certain population threshold are required to produce a qualified rent index. It forms the basis for rent increases during ongoing tenancies and for the application of rent control regulations.