Gender is socially constructed. This volume provides a clear and nuanced demonstration that this also applies to biological sex—a postulate of queer-feminist theories—by examining biological theories for the first time. The natural philosophical and biological-medical theories of gender from different historical periods (Antiquity, early Modernity, present) are presented and linked to social gender orders. Heinz-Jürgen Voss elaborates on the competing positions in detail and shows that, with process-oriented perspectives, many genders can be conceived within biological theories—rather than just two or three.