Occurrence of cancer pain is highly variable and incompletely understood. Surveys are not population-based, are characterized by small heterogeneous samples, and provide sketchy data on etiology of pain, pain syndromes, and clinical or demographic factors. Moreover, the multiple dimensions of pain have not been thoroughly elucidated. Cancer-related pain is caused by the following: 1) direct tumor involvement, 2) diagnostic/therapeutic procedures, and 3) side effects or toxicities of treatment. Individuals may have more than one type of cancer-related pain simultaneously. Occurrence rates range from 14% to 100%, and between 33 and 50% of patients experience pain at some point in the cancer trajectory. Rates are higher (e.g., 70%-100%) in palliative care or pain management settings. Few researchers have focused solely on specific causes of pain in specific patient populations, and only a few included groups at higher risk for pain, such as the elderly or underserved. In general, researchers have not followed pain over time to determine its course, severity, effects on quality of life and functional status, and other related factors. Future researchers should aim to acquire specific information on occurrence of pain in a variety of cancer diagnoses, settings, and groups of people. Longitudinal designs, in-depth exploration of dimensions of pain, and delineation of specific issues in groups such as the elderly, children, and vulnerable populations are essential. Addressing these critical areas will improve our understanding of the occurrence and effects of pain and enhance our ability to provide appropriate interventions, thereby increasing patients' quality of life.