Assent to medical research or treatment may be an intuitively attractive way to address the area between incapacity and capacity that might otherwise be subject to a best interests assessment. Assent has become a widely disseminated concept in law, research, and clinical ethics, but little conceptual work on assent has so far occurred. An exploration of use of assent in treatment and research in children and people with dementia suggests that at least five claims are made on behalf of assent. (...) Since at least some of these may lead to tensions with others, assent requires firmer conceptual underpinning. Whether assent remains primarily a local approach to research in children in the United States, where it appears to fit with legal background conditions, or develops its promise to overturn the dominant, binary, approach to (in)capacity will depend on the strength of future philosophical work to improve the theoretical foundations to assent. (shrink)