Most students are familiar with the idea that XY individuals are destined to become male, and XX individuals are destined to become female, but Nature presents a great diversity of patterns that don’t follow this rule. Some of these differences show up in other species, where we find sex-changers, cross-dressers, and hermaphrodites, among other things. And even among humans, although most people clearly develop as biologically male or female, intersexual characteristics can result from a number of circumstances. This talk explores sex determination, and how we’ve arrived at our current understanding of it.