In order to provide successful interventions to increase physical activity among inactive older adults, it is imperative to understand motivational factors influencing exercise. The authors present data from 191 (baseline) and 125 (12-month) community-dwelling men and women with mean ages of 68.71 (7.47) and 67.55 (7.55) years, respectively, from a strength-training trial. Approximately 53% had diagnosed knee osteoarthritis. Using a Likert scale, participants self-reported their degree of motivation from personal, social, and environmental factors. Using multivariate analyses, the authors evaluated demographic and clinical correlates of motivational factors to join and continue with exercise. The following results were reported: Knee osteoarthritis was positively related to motivation from an organized exercise opportunity and from efficacy/outcome expectations, and knee pain was positively related to motivation from social support and experience with the exercise task. Understanding these motivators might help in targeting recruitment efforts and interventions designed to increase physical activity in older adults with lower extremity arthritis.