Adequate scientific data link regular physical activity with a reduction of the risk of cancer as well as of cancer mortality rates. Regular physical exercise can help prevent many types of cancer and reduce the progression of the disease, improving the clinical outcomes after diagnosis. Conversely, the disease itself and the anti-cancer therapies can affect health-related fitness parameters and the patients may experience side effects that limit their overall functional capacity and exercise ability. While it is important not to overlook the most common toxicities associated with anticancer therapies, there is strong scientific evidence that exercise is safe for all types of cancer and these patients should avoid physical inactivity both during and after their treatment. Before designing an appropriate exercise program for cancer patients, it should be made clear, through appropriate general and site-specific assessment, how much their functional capacity has been affected by the disease, during treatment and after its completion. Moreover, before assessing the patient's physical fitness, or before designing an exercise program, the patient's medical history, chronic comorbidities, and other health problems, as well as any contraindications to exercise should be considered.