In this article, we develop a continuum mechanical model to simulate deformation and phase transformation processes in shape memory alloys. The model is based on a detailed description of the stored energy. Furthermore, the energy dissipation due to phase transformations is taken into account via the maximum-dissipation principle. The results from the 3D numerical simulations of stress induced transformations from the cubic to the tetragonal phase and martensitic variant reorientations in NiMnGa are compared with laboratory experiments on NiMnGa [001]-oriented single crystals.