The thesis is divided in three parts. The first part investigates the notion of reduction in science, the second part focuses on the question of reductionism in the philosophy of mind, and the last part is concerned with the relationship between functional and mechanistic explanation in cognitive science. After making several distinctions inherent to the notion of reduction, four main models of reduction in science are reviewed in the first part of the thesis - Nagelian reduction, Schaffner’s model, structuralist model and new wave reductionism. The second part of the thesis is concerned with the dispute over reductionism in the philosophy of mind. After an overview of the mind-brain identity theory, the anti-reductionist argument from multiple realization is presented, together with its critiques which include arguments against the incompatibility of multiple realization and reductionism and arguments that question the very truth of multiple realization. The last, third part of the thesis focuses on two types of explanation in cognitive science – functional and mechanistic – and considers their relationship.