Gender issues are now receiving more attention on global and national health agendas. However, the evidence base for policy and practice in this area remains limited and conceptual confusion is still common. This article reviews the challenges facing epidemiologists and other researchers who aim to make their work more "gender sensitive." It begins by exploring the concepts of biological "sex" and social "gender" and assesses their implications for the health of both women and men. It then reviews a range of strategies for mainstreaming sex and gender into health research. The article concludes with brief comments on the links between gender equity and wider equality concerns.