Cannabis has been used for centuries for recreational, medical, and industrial use. As the adverse and untoward effects were being recognized in the earlier part of the twentieth century, cannabis use was restricted and eventually considered a substance of abuse. With a resurgence on cannabis research related to medical use and the legalization of cannabis, there is a need to understand the medical utility of cannabis. Practitioners need to be aware of the uses, side effects, contraindications, and appropriate dosing. This chapter provides a general introduction on cannabis for medical use that sets the stage for the rest of the book. The chapter concludes with the case of using cannabis in oncology patients.