The difference between Gender and Sex is one of the founding principles of modern understandings of gender. This principle states that gender identity - the internal psychological experience of gender - is separate from, and not necessary aligned with, the physical sex characteristics of your body. This can be difficult to understand for those whose gender identity matches their assigned sex (cisgender) but can result in discomfort and dysphoria for those whose internal experience of gender does not match how they are viewed by others and society (transgender).