The present thesis is based on a fieldwork that was conducted in a small Mandinka village in the West African republic of The Gambia. This thesis revolves around the Mandinka concept of booroo. Booroo is commonly translated as “medicine”, but the application of booroo is not confined to prevention and treatment of sickness. Booroo refers to diverse substances, objects and, on occasion to certain utterances: such as biomedicines, amulets and prayers which can be used in order to attain changes, for instance to cure sickness, find a spouse or achieve success. Even if the main focus of the thesis is on booroo, which does embrace notions of medicine, the focus is not exclusively on themes prevalent within medical anthropology. This study seeks an understanding of a local concept of power (semboo) through exploring practices related to the use of booroo. The thesis explores how people’s conceptions of power influence their use of booroo. It further investigates what renders booroo and practitioners of booroo powerful. The main part of the thesis discusses power as a transformative force inherent in the world and beings within it, and it also considers how power is perceived as working in social relations. This power may cause wanted and unwanted changes. People can gain knowledge (londoo) of how to manipulate this power, for instance by using booroo. Such knowledge makes people capable of affecting the course of events and renders them powerful in relation to others.This study illustrates how vernacular concepts of power do not simply, or necessarily, give power to those who control power, but also how these understandings shape people’s ways of acting in the world. In this respect, it contributes to the ethnography of medical systems, and additionally, of power, social organisation and religion.