Pain is a subjective perception that is influenced by psychosocial and behavioral factors and physical pathology. In cancer, the source of the pain may be the disease itself, the treatment, or co-occurring pain syndromes. Often, cancer is a progressive disease, and pain may be marked by exacerbations, additional treatment, and remissions. Thus, pain assessment must become part of routine care. Ratings of pain should be performed on a regular basis, just as vital signs are taken on a regular basis. Unlike the other vital signs, however, pain can only be assessed by the patients' verbal and nonverbal behavior. Therefore, it is necessary to actively involve the patient in the assessment process. In deciding what to assess about the pain and how, the clinician needs to balance the purposes of the evaluation with the patient's capacity. During initial assessment it may be possible to include a more comprehensive evaluation of the patient and his or her pain. Relatively brief measures may be used on a routine ongoing basis. When a new type of pain or exacerbation of pain is identified, additional attention beyond pain severity and location may be appropriate. For the very ill patient, it may be possible only to ask a few questions and to observe his or her behavior. In some circumstances, such as when patients are unwilling or unable to report on their pain, it is useful to gather information from caretakers. At a minimum, the severity, location, and pattern of pain and patients' functional activity and mood should be assessed. Timely, appropriate, and thorough assessment and treatment of cancer patients experiencing pain should reduce their suffering and improve the quality of their lives.