Osteoarthritis by definition involves a progressive degeneration in one or many synovial joints and has a multifaceted etiology. The limitation of pathologic responses to an initial insult minimizes the value of microscopic examination of the tissues long after the original injury. The cornerstone of diagnosis has been the plane film x-ray examination, which allows the study and comparison of macroscopic changes that are common to osteoarthritis and aids in diagnosing other causes of arthritis. Synovial fluid analysis in doubtful diagnoses is useful in ruling out specific arthritides such as infection or crystal induced arthritis, or in pinpointing inflammation as a factor in the individual case. Synovial fluid studies are especially important in allowing recognition of conditions in which appropriate treatment can prevent secondary osteoarthritis, e.g., sepsis. This technique may also be necessary to rule out sepsis before implant joint reconstitution.