The energy transition, which entails a fundamental change in the energy supply system, creates new challenges for transmission system operators. In the past, a few large power plants in the transmission grid were responsible for the central provision of energy. Due to the expansion of renewable energies, the number of decentralized power generation facilities is increasing rapidly. These feeders, consumers or storage facilities are mainly connected to the distribution networks. Due to the large number of wind power- and PV systems with mostly weather-dependent generation, a variety of challenges have to be solved. Existing processes must be changed and new technical solutions created. Above all, however, distribution network operators are playing an increasingly important role in the security of electricity supply. At the same time, intensive coordination between distribution and transmission system operators is also becoming increasingly important. This publication is dedicated to the future role of transmission system operators in a flexible energy world and provides recommendations for action.