Although the typical subtypes of embryonal, botryoid, alveolar, and pleomorphic rhabdomyosarcoma are easily recognized by simple light microscopic examination, poorly differentiated forms are difficult to diagnose and classify. The availability of markers connoting simple myogenous or skeletal muscle differentiation has contributed tremendously to the diagnosis of even poorly differentiated rhabdomyosarcoma. Definition of typical and minimal ultrastructural criteria has also been instrumental. Proper classification of rhabdomyosarcoma into subtypes with prognostic significance has been a difficult task. Although most pathologists have accepted the original classification scheme of embryonal, botryoid, alveolar, and pleomorphic rhabdomyosarcoma, they have used variable histologic criteria to define the various subtypes. Most of the major discrepancies occurred in the definition of embryonal versus alveolar rhabdomyosarcoma, especially in those cases that required establishment of minimal criteria. For example, poorly differentiated round-cell rhabdomyosarcoma has frequently been erroneously classified as embryonal, because of the absence of an alveolar pattern. This variety has been recognized as a form of alveolar (solid alveolar) rhabdomyosarcoma by the National Cancer Institute (NCI) scheme. Comparative analysis of several proposed classification schemes has led to the establishment of well-defined criteria for the major subtypes of rhabdomyosarcoma and to the recognition of subtypes with prognostic significance.