Persons with chronic progressive neurologic diseases such as Parkinson disease (PD) and multiple sclerosis (MS) face significant declines in mobility and activities of daily living, resulting in a loss of independence and compromised health-related quality of life over the course of the disease. Such undesirable outcomes can be attenuated through participation in exercise and physical activity, yet there is profound and prevalent physical inactivity in persons with PD and MS that may initiate a cycle of deconditioning and worsening of disease consequences, independent of latent disease processes. This Special Interest article highlights the accruing evidence revealing the largely sedentary behaviors common among persons living with physically disabling conditions and summarizes the evidence on the benefits of physical activity in persons with PD and MS. We then examine the social cognitive theory as an approach to identifying the primary active ingredients for behavioral change and, hence, the targets of interventions for increasing physical activity levels. The design and efficacies of interventions based on the social cognitive theory for increasing physical activity in persons with PD and MS are discussed. Finally, a rationale for adopting a secondary prevention approach to delivering physical therapy services is presented, with an emphasis on the integration of physical activity behavior change interventions into the care of persons with chronic, progressive disabilities over the course of the disease.Video Abstract available (see Video, Supplemental Digital Content 1, http://links.lww.com/JNPT/A42) for more insights from the authors.