We humans are diverse. But how to understand human diversity in the case of cognitive diversity? This Element discusses how to properly investigate human behavioural and cognitive diversity, how to scientifically represent, and how to explain cognitive diversity. Since there are various methodological approaches and explanatory agendas across the cognitive and behavioural sciences, which can be more or less useful for understanding human diversity, a critical analysis is needed. And as the controversial study of sex and gender differences in cognition illustrates, the scientific representations and explanations put forward matter to society and impact public policy, including policies on mental health. But how to square the vision of human cognitive diversity with the assumption that we all share one human nature? Is cognitive diversity something to be positively valued? The author engages with these questions in connection with the issues of neurodiversity, cognitive disability, and essentialist construals of human nature.