Sex and gender are essential, inalienable characteristics of the human being permeating the biological, psychological, interpersonal, social, transcendental aspects of our existence. Human sexual development takes place in the context of physical, emotional, cognitive, and social development, beginning at conception and continuing throughout life. It is influenced by biological maturation and complex interactions of psychological, interpersonal, cultural factors and (physical) environmental conditions. Using a developmental-psychopathology approach, we summarise our under standing of the life course-specific features of sexual development, placing them in the broader context of development. We will address gender differences, the development of gender-related concepts, sexual orientation, gender identity, and the development of sexual behaviour at different stages of life. In the field of sexual development and gender, science is often pushed to its limits. It is important that professionals express their opinions and make their decisions with due caution and objectivity.