Small changes in delivered radiation dose may have a dramatic effect on the probability of treatment success or complications. Also, radiation therapy is stressful for animal patients, and the treatment procedure results in the consumption of many resources making it expensive for the pet owner. Thus, it is critical that individuals administering radiation therapy to pets with cancer have a sound understanding of the basic physical principles of matter and ionizing radiation, as well as an appreciation for the nuances of treatment planning. In this article, these topics are reviewed and illustrations are provided to aid in the understanding of important principles.