Pain is a complex phenomenon that has physiological, psychological and social factors that influence the individual patient experience. It is subjective so the patient's perspective is important to understand. It has both a physical and an affective (emotional) component. To reflect this, the International Association for the Study of Pain (IASP [106]) published the following definition of pain: ‘An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage’. As pain is subjective, another favoured definition for use in clinical practice, proposed originally by McCaffrey ([142]) and cited in McCaffrey ([143], p. 2), is: ‘Pain is whatever the experiencing person says it is, existing whenever the experiencing person says it does’.

Pain in patients with cancer is often not a purely physical experience but involves many other factors. Pain may have psychological, physical, social and spiritual components.