Compressible flows with phase transition of fluid components are known to be very sensitive, especially if phase transition and the associated latent heat release develop in flow regions close to Mach number unity. Then thermal choking causes additional steady or moving shocks, socalled "condensation shocks". In experiments we detected new selfexcited instabilities with higher order bifurcation and sudden frequency jumps. Extensive numerical simulations have confirmed that viscosity effects like boundary layers, separation etc. are not relevant, but interaction of compressibility waves with the local rate of heat addition. Internal transonic two-phase flow problems have been investigated, e.g. the condensing flow through axial cascades of low pressure steam turbines including various nucleation models and forced excitation due to rotor/stator interaction. All numerical simulations are performed by applying our inhouse code PHASE CD.