Abstract: Seeking children’s assent has been put forward as a way to foster children’s involvement in the healthcare decision-making process. However, the functions of the concept of assent within clinical care are manifold, and methods used to recognize children’s capacities and promote their involvement in their care remain debated. We performed an instrumentalist concept analysis of assent, with 58 included articles. Final themes were jointly identified through a deliberative process. Two distinct perspectives of assent were predominant: as an affirmative agreement for a specific decision and as part of a continuous, interactive process of care. Differing standards were provided as to how and when to apply the concept of assent. The concept of dissent was largely omitted from conceptions of assent, especially in situations for which children’s refusal would lead to severe health consequences. Ethical implications included fostering autonomy, reducing physical/psychological harm to the child, respecting the child as a human being, and fulfilling the universal rights of the child. There remain important gaps in the theory of assent and its desirable and possible practical implications. Practical standards are largely missing, and evidence supporting the claims made in the literature requires further investigation.