Most introductory and many advanced biology textbooks describe mammalian sexual differentiation as a process regulated, in males, by the Y chromosome and the actions of androgenic hormones. Typically, female differentiation is described as being the result of the absence of these factors. In essence, the female phenotype is described as the default form. IN this presentation I will show how this model is out-of-date with current research findings and how sexual differentiation in both males and females is due to a complex interaction of biological factors. Discussion will focus on how we can incorporate this changing paradigm into our courses.