Medical marijuana continues to gain acceptance and become legalized in many states. Various species of the marijuana plant have been cultivated, and this plant can contain up to 100 active compounds known as cannabinoids. Two cannabinoids seem the most clinically relevant: Δ9-tetrahydrocannabinol (THC), which tends to produce the psychotropic effects commonly associated with marijuana, and cannabidiol (CBD), which may produce therapeutic effects without appreciable psychoactive properties. Smoking marijuana, or ingesting extracts from the whole plant orally (in baked goods, teas, and so forth), introduces variable amounts of THC, CBD, and other minor cannabinoids into the systemic circulation, where they ultimately reach the central and peripheral nervous systems. Alternatively, products containing THC, CBD, or a combination of both compounds, can be ingested as oral tablets or via sprays applied to the oral mucosal membranes. These products may provide a more predictable method for delivering a known amount of specific cannabinoids into the body. Although there is still a need for randomized controlled trials, preliminary studies have suggested that medical marijuana and related cannabinoids may be beneficial in treating people with chronic pain, inflammation, spasticity, and other conditions seen commonly in physical therapist practice. Physical therapists, therefore, should be aware of the options that are available for patients considering medical marijuana and should be ready to provide information for these patients. Clinicians also should be aware that marijuana can produce untoward effects on cognition, coordination, balance, and cardiovascular and pulmonary function and should be vigilant for any problems that may arise if patients are using cannabinoids during physical rehabilitation.