The transformation in dicalcium silicate has been studied by a combination of optical microscopy and transmission electron microscopy (TEM). The -phase forms as long needles or laths whose long axis is parallel to [001] . These tend to occur in bundles. TEM shows that regions of in partially transformed specimens are not internally twinned but contain a high density of dislocations and of low angle grain boundaries running parallel to the long axis of the particles. particles can be induced to form by the stress near a microhardness indentation. The nature of the transformation and the effect of the very large dilatation (14%) which accompanies the transformation are discussed. The transformation has been studied on a TEM hot stage. It is confirmed that (100) twins are formed on cooling through the transformation temperature. Local stresses set up at the intersection of twin lamellae with domain boundaries are relaxed by slip on (100).