This article provides a critical introduction to contractualism as a moral or ethical theory, that is, as a theory of the rightness and wrongness of individual conduct – focusing specifically on the influential 'Kantian' version of contractualism due to T. M. Scanlon. I begin by elucidating the key features of Scanlon's contractualism: justifiability to others; reasonable rejectability; the individualist restriction; and mutual recognition. I then turn to discuss both its appeal and the main objections that have been raised to it (...) – objections concerning our duties to the cognitively limited and impaired, aggregation, demandingness, normativity and explanatory adequacy. I conclude by mentioning some contractualist alternatives to Scanlon's theory. (shrink)