Stresses are developed internally in metals when a change in density and strength arises from a phase transformation. It is shown that plastic flow, generally confined to the weaker phase, results from the accommodation of strain due to the transformation front. The form of the plastic flow is considered in terms of the extreme cases of yield and creep behaviour. It is deduced that, for a complete cycle both ways through the transformation temperature, the resultant deformation varies linearly with the applied stress (provided this is small), the fractional volume change on transformation and inversely as the flow stress of the weaker phase. The deformation is not zero in the absence of external stress except where the phase transformation front has random orientation and movement. The theoretically deduced relations are examined experimentally by observing the deformation of specimens with attached weights, giving small tensile stresses, while their temperature was cycled through a transformation point. Phase transformations were examined in a number of metals involving a variety of crystal structures: reasonable agreement between theory and experiment was obtained in all cases.