In this paper, we offer a new account of disability. According to our account, some state of a person's biology or psychology is a disability if that state makes it more likely that a person's life will get worse, in terms of his or her own wellbeing, in a given set of social and environmental circumstances. Unlike the medical model of disability, our welfarist approach does not tie disability to deviation from normal species' functioning, nor does it understand disability in essentialist terms. Like the social model of disability, the welfarist approach sees disability as a harmful state that results from the interaction between a person's biology and psychology and his or her surrounding environment. However, unlike the social model, it denies that the harm associated with disability is entirely due to social prejudice or injustice. In this paper, we outline and clarify the welfarist approach, answer common objections and illustrate its usefulness in addressing a range of difficult ethical questions involving disability.