The various methods of determination of the clonality of human tumors are described. There are three major approaches based on X-chromosome inactivation analysis, lymphocyte analysis, and somatic mutation analysis. For each of these approaches there are established methods and more recent methods based on DNA analysis. The increasing number of methods available increases the scope of clonality determination to most tumors. All the methods have inherent advantages and disadvantages, and these are discussed in relation to their clinical application.