Anaesthesia is nowadays seldom accomplished by a single agent because no single agent is able to provide all components of anaesthesia without seriously compromising haemodynamic and/or respiratory function, reducing operating conditions, or postponing postoperative recovery. Because of the small therapeutic window a detailed characterisation of the concentration-effect relationships of anaesthetic agents is required to allow a proper selection of the various intravenous agents and the combinations thereof to an optimal therapeutic pharmacological effect in the absence of significant side effects. During the past decade, for propofol and the various opioids fentanyl, alfentanil, sufentanil, and remifentanil considerable progress has been made in the characterisation of the pharmacokinetics and pharmacodynamics of these agents and of the combinations thereof. This manuscript describes the pharmacokinetic and dynamic interactions between these agents and the determination of optimal concentrations that assure adequate anaesthesia and a rapid return of recovery.