Background The Delphi technique is an approach with much to offer health-care research. It was first devised by the RAND corporation in the 1950s and later, in the 1970s, was rediscovered by the nursing profession. Delphi is one of a range of consensus techniques, all of which fall under the broad classification of action research approaches. The purpose of the Delphi technique is to test opinion consensus amongst a group of experts. Content A brief overview of the various consensus methods is given. The development and operation of the Delphi technique is then considered and a critical assessment of the technique is offered. Conclusions Delphi is potentially useful across a wide range of applications, however, it is important to appreciate the strengths, weaknesses and pitfalls to avoid in its application.