Cancer patients experience pain in multiple sites and from several pathophysiologies of the symptom complex. The fluctuating nature of cancer pain intensity is a relevant clinical feature and depends on disease patterns and pain mechanisms. Breakthrough pain is defined as episodes of pain that "break through" the control of an otherwise effective analgesic therapy. Traditional ways of classifying pain in the cancer population include distinguishing pain associated with the treatments, the tumor, or unrelated to both and between chronic and acute pain. In focusing on the care of the cancer patient with pain, it is useful to be familiar with the characteristics of the typical syndrome found in association with different tumor types and anatomic locations. An understanding of the etiology of pain in relation to the cancer is useful in recognizing these complications and in treating them. This article reviews the methods presently applied to the classification of cancer pain and highlights the need for more research in this area.