Respiratory depression limits the use of opioid analgesia. Although well described clinically, the specific mechanisms of opioid action on respiratory control centres in the brain have, until recently, been less well understood. This article reviews the mechanisms of opioid-induced respiratory depression, from the cellular to the systems level, to highlight gaps in our current understanding, and to suggest avenues for further research. The ultimate aim of combating opioid-induced respiratory depression would benefit patients in pain and potentially reduce deaths from opioid overdose. By integrating recent findings from animal studies with those from human volunteer and clinical studies, further avenues for investigation are proposed, which may eventually lead to safer opioid analgesia.