This chapter outlines psychological understandings of sexuality charting differences in definitions and expression of sexual orientations and practices between traditional and critical schools of thought. Theoretical representations of sexuality as the product of biological processes and substances (evolution, hormones, anatomy, etc.) and heterosexuality as the most natural and normal mode of sexual identity and behaviour are challenged through the introduction of critical perspectives. Drawing from sociological, feminist and "queer' theory perspectives, the social construction of sexuality is emphasised with issues of power, discourse and resistance highlighted and the complexity and fluidity in sexed identities and relationships detailed.