Synovial fluid is defined as the collection of fluid confined within a joint space. Synovial fluid is physiologic and acts as a joint space lubricant for articular cartilage and a nutrient source through diffusion for surrounding structures, including cartilage, the meniscus, and the labrum. Synovial fluid is produced as an ultrafiltrate of blood plasma and is primarily composed of hyaluronan, lubricin, proteinase, collagenases, and prostaglandins. Synovial fluid production is from fibroblast-like type B synovial cells. Physiologic changes in synovial fluid volume and content occur in response to trauma, inflammation, and bacterial, fungal, or viral penetration. When patients present with acutely painful joints with suspicion of infection, inflammation, or noninflammatory causes of effusion, synovial fluid aspiration and analysis are imperative to aid in diagnosis and direct treatment modality.