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Opioid analgesics are the mainstay in the treatment of chronic pain associated with active cancer (World Health Organization (WHO), 2019) and other chronic progressive illnesses, particularly in the context of advanced disease. Although concurrent use of other approaches and interventions may be appropriate in many patients, and necessary in some, an opioid is a preferred approach in almost every patient with moderate or severe chronic pain. The safe and effective use of opioid drugs is a key competency in palliative care and is predicated on a continual effort to balance expected benefits against the potential for harm. This chapter focuses on the evolving science, along with safe clinical practice of opioid analgesia, with a view to informing clinical practice and future research. Opioid analgesics are conventionally distinguished from nonopioid analgesics, such as paracetamol and the non-steroidal antiinflammatory drugs, and the so-called adjuvant analgesics. The latter category includes drugs such as the glucocorticoids, analgesic antidepressants, and gabapentinoids that have primary indications other than pain but are effective analgesics in specific circumstances. Opioids have a complex pharmacology, which is reflected in great inter-individual variability in the response to different opioid compounds. Recent evidence has established the importance of genetic variability in determining not just the response to exogenous opioids but also the response to endogenous opioids involved in the inherent ability to attenuate pain transmission (Lee et al., 2014; De Felice, 2016). Concurrently, advances in neuroimaging have helped to model the …