An essential feature of the malignant state is the clonal expansion of a single cell (Fialkow, 1976). This discovery has had a great impact upon our understanding of the development and progression of tumors. The presence of a clonal population of cells often has been ascertained by the detection of acquired karyotypic abnormalities in the malignant cells (Rowley, 1978), or the altered pattern of expression of the enzyme G-6-PD in appropriate heterozygous females (Fialkow, 1976). The introduction of the techniques of molecular biology has provided the investigator with a number of other means for detecting clonal populations of malignant cells (Korsmeyer et al., 1983); this has been amply demonstrated for malignancies involving lymphocytes (Korsmeyer et al., 1983; Arnold et al., 1983; Sklar et al., 1984). In this review we will describe the genes and the probes that have been used to determine the clonal nature of malignancies involving T lymphocytes and indicate, where possible, how these studies have altered our understanding of these diseases.