By using a high speed rotating mirror camera combined with a microscope, the rapid initial lengthening of twins and their subsequent broadening have been studied. In zinc, twins of up to 1$\cdot$9 mm appeared in crystal surfaces within 1 $\mu$s and subsequently the tips of such twins were observed to move at up to 780 m/s. It is pointed out that observations of twins which simply appear in crystal surfaces do not necessarily yield information on the true twin propagation speed in the twin plane. However, such information has been derived from experiments in which the twin growth was followed from the point of twin nucleation located under a knife edge. Increasing the rate of loading applied to the knife edge by more than an order of magnitude did not significantly alter the maximum speed of growth. Therefore the maximum speed recorded in experiments using a knife edge, about 600 m/s, is thought to be close to the maximum true twin tip speed in zinc in the twin plane. It is approximately one third of the slowest sound speed in the twinning direction. Broadening of twins under a steadily increasing stress is a smooth process which, in a specimen, may occur in several twins at once and may involve the movement of either or both twin boundaries. The present investigation has shown twins to broaden at speeds of up to 35 m/s. These are one and often two orders of magnitude higher than those noted during previous investigations. By using quartz piezo-electric pressure transducers it has been possible to measure the stresses applied to crystals whilst the formation of twins in surfaces of the crystals was being recorded. Comparison with theory has shown that in one example a twin broadened at a speed more than an order of magnitude greater than that predicted on the basis of the Cottrell-Bilby mechanism. The nucleation of a twin or twins near the tip of a rapidly lengthening twin has been noted in many crystals. This nucleation occurs on a plane close to and parallel to the composition plane of the lengthening twin. A brief study of the kinetics of twinning in $\beta$-tin has shown that twins broaden at rates similar to those occurring in zinc. Detwinning of tin crystals has been observed to occur at speeds similar to those in twinning, emphasizing the reversibility of the twinning process.