It has always been recognised that the practice of medicine requires an ethical base. This ethical base also provides a backbone for a legal framework. Until recently paternalism was the accepted norm in the physician-patient relationship. Based on knowledge and experience the physician decided what treatment was in the best interests of the patient. However, in recent years medicine has changed from a predominantly paternalistic profession to one that is more patient centred. The physician informs and advises the patient but it is the patient who makes the decision. This is reflected in the change to principle-based medical ethics. The central concept in principle-based medical ethics is patient autonomy. This paper explores the application of autonomy and the legal requirement of informed consent to the clinical practice of obstetric anaesthesia. Autonomy requires that a patient has capacity to make a decision. Whether labouring women have capacity is discussed. There are times when patients are not able to act autonomously and the roles of advance directives (which can include birth plans) and substitute decision makers become important. The application of ethical practice to research in obstetric anaesthesia is considered.