The importance of various surface imperfections in the nucleation of ice crystals iR considered and it is concluded that dislocations are not preferred nucleation sites, whereas steps and re-entrant corners do encourage nucleation. Expressions are derived for the height of the free energy barrier in several important cases, and the retention of embryos under unsaturated conditions is discussed. Embryos initially grow by twodimensional nucleation, but ultimately a dislocation growth mechanism becomes more important. There is not yet enough experimental evidence to decide upon the reality of the entropy effect, previousl:y- proposed, which inhibits nucleation upon certain CITRta] faces.