Prosecutions for sexual assault most often focus on whether the Crown has proven that the complainant did not consent to the sexual activity in issue, based on her subjective state of mind at the time of the offence. However, Canadian criminal law also provides that no consent is obtained where the complainant is incapable of consenting. In cases where the complainant has a mental disability affecting cognition or decisionmaking, prosecutors in Canada have been reluctant to argue that the complainant was incapable of consenting. In this article, the authors agree that claims of incapacity should be used sparingly, but contend that the doctrine of incapacity may be applicable and useful in some cases where the accused has exploited the complainant’s disability. They argue that capacity to consent to sexual activity should be defined situationally, rather than as an all-or-nothing measure. Since consent is given to a specific person in a specific circumstance, incapacity should be also assessed by reference to the particular context of the case. This approach to incapacity has been adopted in English and American cases, which provide examples of how it might be applied and understood in Canada. A situational definition of incapacity offers some legal recognition of the particular challenges faced by women with mental disabilities with respect to sexual abuse, without disqualifying them from any lawful sexual activity in other contexts.