There is now widespread acknowledgement of the absence of a sound evidence base underpinning many of the decisions made in primary care. Randomised controlled trials represent the methodology of choicefor determining efficacy and effectiveness of interventions, yet researchers working in primary care have been reluctant to use intervention studies, favouring observational study designs. Unfamiliarity with the different trial designs now available, and the relative advantages and disadvantages conferred by each, may be one factor contributing to this paradox. In this paper, we consider the principal trial designs available to primary care researchers, discussing the contexts in which a particular design may prove most useful. This information will, we hope, also prove useful to primary care clinicians attempting to interpret trial findings.