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The newly developed multiphase high-strength steels with improved formability characteristics mainly owe their beneficial properties to their special microstructural composition. The multiphase structure can in general be classified as a mixture of varying amounts of soft steel phases and distinctly harder ones. As introduction the variety of nowadays multiphase steels is mentioned concerning the process strategy and the resulting range of strength. Focussing on the concept of TRIP steels (TRIP = transformation induced plasticity) the retained metastable austenite as one special steel phase plays an additional role. A microstructural model, which was originally applied to dualphase steels, was successfully used to explain the work hardening of TRIP steels at higher strain rates. The multiphase structure is also interesting in terms of void nucleating mechanisms. As indicated by microstructural observations austenite particles play a double role depending on the grain size.