Magnetic shape memory alloys have attracted a lot of excitement, since they allow shape changes of more then 10% with a frequency in the kHz regime - a property which is known to be related to a martensitic phase transition. The material system Ni2MnGa is the most promising candidate for applications, but its operation temperatures and ductility still need to be improved. For this purpose an extension of the currently very limited knowledge on the phase diagram and the nature of the transitions is decisive . We were able to describe the phase transition in detail, to reveal the involved delicate interplay of vibrational and magnetic excitations and to accurately predict the transition temperature . The concepts can be transferred to martensitic phase transitions in steels, playing an important role for deformation processes such as TRIP (transition induced plasticity).