The prospect of a justification defence in cases of direct sex discrimination is universally criticised by academic commentators on the ground that it would subvert the goal of equality that underlies sex discrimination and equal treatment legislation. At the outset the thesis examines the differences between the sexes, how these differences can be used to explain the distinction between direct and indirect sex discrimination and considers various concepts of equality. Building on various elements of the existing justification defences for indirect sex discrimination and disability discrimination, this thesis constructs a model justification defence. The impact on equality of such a defence is assessed by reference to the main existing legislative exceptions for direct sex discrimination and various judicial exceptions that have been created, in the main, by the European Court of Justice. Further, the thesis considers whether the blanket prohibition against the use of sex stereotypes is warranted and the extent to which they might be permitted under the model defence. The conclusions reached are that criticism of the potential defence is overstated. Rather than undermining the goal of sex equality, such a defence could in fact enhance the degree of legal protection as long as the criteria of the defence are stringently drawn. Indeed, in relation to some areas of direct sex discrimination, for example pregnancy and maternity, the introduction of such a defence could enhance the degree of equality. Moreover, the introduction of such a defence could introduce a greater degree of openness and clarity into this complex area of law.