alstria office REIT-AG is a real estate investment trust engaged in the acquisition, management, refurbishment, and sale of office properties throughout Germany. The vast majority of the company's real estate portfolio is dispersed between the German regions of Rhine-Ruhr, Hamburg, Rhine-Main, and Stuttgart. The company derives nearly all of its revenue in the form of rental income from the leasing of its properties. alstria's largest tenants in terms of annual rental revenue include a variety of local government, automotive, telecommunications, and insurance entities. The company focuses on a small number of large tenants that account for the majority of its rental income.