Objective. To systematically review studies describing the course of functioning in patients with osteoarthritis (OA) of the hip or knee and identifying potential prognostic factors. Methods. A systematic search was performed. Studies involving patients with hip or knee OA, >6 months of followup, and outcome measures on functional status or pain were included. Methodologic quality was assessed using a standardized set of 11 criteria; a qualitative data analysis was performed. Results. Approximately 6,500 titles and abstracts were screened and 48 publications were considered for inclusion. Eighteen studies, 4 of which met the high methodologic quality criteria, were included. For hip OA, there was limited evidence that functional status and pain do not change during the first 3 years of followup. After 3 years, however, a worsening of functional status and pain was seen. For knee OA, there was conflicting evidence for the first 3 years and limited evidence for worsening of pain and functional status after 3 years. Furthermore, limited evidence was established for negative associations between future functional status and laxity, proprioceptive inaccuracy, age, body mass index, and knee pain intensity. In contrast, greater muscle strength, better mental health, better self-efficacy, social support, and more aerobic exercise were protective factors in the first 3 years. Conclusion. Pain and functional status in hip or knee OA seem to deteriorate slowly, with limited evidence for worsening after 3 years of followup. In specific subgroups, prognosis in the first 3 years of followup was either worse or better, as both risk factors and protective factors were identified. Prognostic factors included biomechanical factors, psychological factors, clinical factors, and treatment modalities. To strengthen the evidence, further high-quality longitudinal research on hip or knee OA functioning is needed.