This chapter deals primarily with intravenous (IV) patient-controlled analgesia (PCA), although PCA via other systemic routes. Opioids having very short or very long durations of action are not usually recommended for use in PCA, at least for general ward use. The chapter focuses on the use of a single opioid with PCA. Much less often, combinations of an opioid with another drug are used. The bolus dose is the amount of opioid that the PCA machine will deliver when the demand button is pressed. The size of the dose can influence the success or otherwise of PCA. Safety and efficacy of PCA require the patient to have an adequate understanding of the technique. Many opioids, including morphine, fentanyl, hydromorphone, oxycodone, tramadol, and pethidine, have been used with PCA. Opioids having very short or very long durations of action are not usually recommended for use in PCA, at least for general ward use.