Osteoarthritis of the hip and knee is a leading cause of functional disability and compromised quality of life in older patients and a significant public health issue. Emerging research shows sex and gender differences in osteoarthritis which, to date, may not be appreciated by the orthopedic community. This article discusses sex and gender differences in osteoarthritis with a focus on disease involving the hip and knee. Understanding what we know (and do not know) about sex and gender differences in this disorder is critical to improving quality of care for our patients.