Patients who cannot be classified as having a well-defined arthropathy typically are labeled as having undifferentiated arthritis. Some patients develop sufficient features to permit classification, whereas others remain undifferentiated, but with persistent joint inflammation, functional disability, and development of radiographic damage. Identifying the subset of patients destined to develop rheumatoid arthritis, spondyloarthropathy, or a more severe and persistent form of undifferentiated arthritis and choosing appropriate treatment strategies remain challenges for clinicians. Numerous investigative strategies are available with which to characterize undifferentiated arthritis and assess prognosis. This article discusses the characteristics of undifferentiated arthritis at presentation and the investigative strategies that can be used to predict prognosis and outcome early in the disease course. Therapeutic strategies also are explored.