Malignancies of thymus-derived (T) lymphocytes can be divided into two major groups: diseases of T cells expressing immature T cell markers (T-ALL, T cell lymphoblastic lymphoma) and diseases of malignant T cells expressing markers in a pattern similar to normal mature T cells (T cell CLL, ATL, various forms of CTCL, and T-PLL). Until specific pathways of normal cell maturation are known, the relationship of phenotypic expression of various T cell markers by malignant T cells to a particular stage of normal T cell differentiation must remain speculative. However, phenotypic characterization of malignant T cells is an important first step in the study of events that transpire in the development of T cell malignancies. Future parallel study into the mechanisms of normal and aberrant T cell maturation will undoubtedly lead to greater understanding of the pathogenesis of the T cell malignancies, and therefore pave the way for specific therapies for these difficult-to-treat syndromes.