The study introduces the reader to one of the most contentious areas of German tenancy law: the modernization of residential space. The tenant's obligation to tolerate improvement measures and the landlord's right to increase rent are at the center of the analysis. Starting from the historical development, the current legal situation is presented comprehensively. Weaknesses and problem areas of the statutory regulations are examined, and potential solutions are proposed. In a separate section of the study, the author focuses on the situation in the new federal states and East Berlin. The transitional law established for this area is analyzed in detail. The results of a survey illustrate the factual and subsequent effects that the special law has triggered. The concluding part of the study offers concrete recommendations for reform.