Eighteen new cases of colour mosaics are described. Previous reports of mosaics are collocated and briefly discussed. Various mechanisms are described which may engender three types of mosaicism. These are the (1) somatic, (2) the soma-gonadic and (3) the gonadic. The first type is the most frequent but representatives of the other two types are not unknown. The analytical study of both spontaneous and induced mosaicism holds promise for the study of cell lineage and tissue dynamics. A possible tendency of white spotting to act as an organiser is noted. The usefulness of the study of mosaicism for analysis of complex loci is touched upon.