The longitudinal dynamics of highly charged bunches circulating in storage rings is still a matter of research especially in view of the interaction of the electrons with their own coherent radiation field. In the talk I will introduce the Vlasov-Fokker-Planck equation which is used for the numerical modeling of the longitudinal dynamics. Instability thresholds and the behavior of the bunch beyond the threshold will be presented and compared to observations at BESSY II and DIAMOND. In the second part of the talk I will present two examples of longitudinal bunch gymnastics: The dynamics of a sliced bunch and the "Variable Bunch Length Storage Ring". The bunch length can be changed freely if two superconducting higher harmonic cavities are installed in the ring which operate at different frequencies. First results will be shown for instability thresholds in this scenario.