The Energiewende is a paradigm change that can be witnessed at latest since
the political decision to step out of nuclear energy. Moreover, despite common
roots in Electrical Engineering, the control community and the power systems
community face a lack of common vocabulary. In this context, this paper aims at
providing a systems-and-control specific introduction to optimal power flow
problems which are pivotal in the operation of energy systems. Based on a
concise problem statement, we introduce a common description of optimal power
flow variants including multi-stage-problems and predictive control, stochastic
uncertainties, and issues of distributed optimization. Moreover, we sketch open
questions that might be of interest for the systems and control community