At times clinicians and pathologists underestimate the value and significance of histopathological diagnostics for synovial tissue. However, the most common diseases of the synovium show specific morphological hallmarks that allow an exact diagnosis. Using the synovitis score allows one to distinguish between degenerative (low-grade synovitis) and inflammatory rheumatic (high-grade synovitis) diseases. Synovial biopsies are not only especially indicated when there are atypical patterns of arthritis, clinical options have been exhausted or monarthritis of unknown origin occurs, but also in patients with known rheumatoid arthritis. Joint infections, crystal-induced arthritis or pigmented villonodular synovitis can also be diagnosed as secondary synovial diseases. Providing clinical information when submitting biopsies/tissue specimens is essential to classify even unspecific morphological changes. Immunohistochemical staining, polarization microscopy or molecular biology techniques (PCR) may be used to ensure diagnoses.