In this thesis, I address the metaphysical question 'What is humour?' and the ethical question 'When is humour immoral?' Consulting a dictionary reveals a circle of definitions between 'amusement', 'funniness', and 'humour'. So I split the metaphysical question 'What is humour?' into three questions: 'What is amusement?', 'What is funniness?' and 'What is humour?' By critically analysing then synthesising recent research in philosophy, psychology and linguistics, I give the following answers: (1) x amuses y if and only if: (i) y is in a non-serious, non-threatened, non-goal-directed state. (ii) y simultaneously activates two incompatible interpretations of x via unsound logic. (iii) (ii) sufficiently increases the psychological arousal of y. (2) x is funny if and only if x merits amusement. (3) x is humour if and only if the function of x is to merit or elicit amusement. Similarly, I split the ethical question 'When is humour immoral?' into three questions: 'When is amusement immoral?', 'How does immorality affect funniness?' and 'When is humour immoral?' By using (1), (2) and (3), I give the following answers: (4) Amusement is immoral when the correct theory of normative ethics defines (1.i), (1.ii) or (1.iii) to be immoral. (5) Immorality negatively affects funniness. (6) Humour-types are not subject to moral evaluation. (7) Humour-tokens are immoral when the correct theory of normative ethics defines them to be immoral. This thesis contributes to the knowledge of the subject in two ways. First, by critically analysing and synthesising recent inter-disciplinary research, (1) provides a more precisely specified definition of amusement than that in the philosophical literature. Second, research on humour, across all disciplines, conflates the three closely related but distinct concepts of humour, amusement and funniness. By providing a linear sequence of definitions, (1), (2) and (3) avoid the conceptual confusions that commonly occur.