The terms ‘gender’ and ‘sex’ are commonly used as equivalents regardless of their significant difference. This ‘misuse’ gives rise to a terminological gap that must be avoided in all areas, but especially in the area of law. This chapter briefly defines the two concepts in the light of the legal practice and the definitions provided by other social sciences. In a nutshell, sex refers to biological differences, whereas gender refers to the cultural and social elements that are expected to be fulfilled by the members of a certain sex. There exist two sexes, ‘female’ and ’male’, and three main categories of gender: ‘feminine’, ’masculine’ and ’androgynous’. The six most common sex and gender combinations are: masculine female, masculine male, feminine female, feminine male, androgynous male and androgynous female.