Analysis of synovial fluid (SF) is among the most useful means of evaluating patients with joint complaints. The best reasons to aspirate a joint include suspicion of infection or crystal-induced arthritis, and generally, the risks of the procedure are acceptably low. While there is no consensus regarding which tests should be considered routine for all SF obtained, data support the performance of a SF white blood cell count with differential, gram stain and culture, and examination for crystals by polarizing microscopy. The separation of SF into groups (noninflammatory, inflammatory, or purulent) should not be relied upon diagnostically. Care must be taken in obtaining and handling joint fluid, and caution applied to interpretation of test results, because over-reliance on any individual test may promote misdiagnosis.