Synovial sarcoma is a rare soft tissue tumor of children and adults that is unrelated to synovium and can occur in almost any part of the body. The familiar biphasic synovial sarcoma has discernible glandular or solid epithelial structures, and monophasic forms have characteristic ovoid or spindle cells with only immunohistochemical or ultrastructural evidence of epithelial differentiation. There are several morphologic patterns, including myxoid and hemangiopericytic, and behaviorally distinct calcifying, ossifying, and poorly differentiated subtypes can be recognized. Most synovial sarcomas are immunoreactive for cytokeratin, epithelial membrane antigen, and bc12 protein, and negative for CD34, and many express S100 protein and CD99 (MIC2). Nearly all synovial sarcomas have a specific t(x;18) (p11.2;q11.2) chromosomal abnormality, resulting in fusion of either of two variants of the SSX gene with the SYT gene; the genetic features may relate to morphology and outcome. The differential diagnosis can include a wide range of spindled, polygonal, or round cell sarcomas. Clinically, there have been marked recent improvements in local control of disease and lesser ones in management of metastases. The pathology, differential diagnosis, and behavior of this unique tumor are reviewed.