Poorly controlled cancer pain is a significant public health problem throughout the world. There are many barriers that lead to undertreatment of cancer pain. One important barrier is inadequate measurement and assessment of pain. To address this problem, the Pain Research Group of the WHO Collaborating Centre for Symptom Evaluation in Cancer Care has developed the Brief Pain Inventory (BPI), a pain assessment tool for use with cancer patients. The BPI measures both the intensity of pain (sensory dimension) and interference of pain in the patient's life (reactive dimension). It also queries the patient about pain relief, pain quality, and patient perception of the cause of pain. This paper describes the development of the Brief Pain Inventory and the various applications to which the BPI is suited. The BPI is a powerful tool and, having demonstrated both reliability and validity across cultures and languages, is being adopted in many countries for clinical pain assessment, epidemiological studies, and in studies of the effectiveness of pain treatment.