The thesis examines the concept of the supernatural beings called álfar, elves . It aims to achieve a thorough understanding of their image as it appears in the Scandinavian written sources from up till the 15th century. The focus of the thesis is on the various aspects of the concept which are revealed in the sources. I problematize the variation in the use and understanding of the concept and identify the factors responsible for this variation. The thesis is organised around a chronologically traceable line of the semantic transformations of the concept, and considers the effects of the changing cultural context upon it.My main question is: How is a pre-Christian mythological concept transformed in the changed context of a Christian culture? I come to the conclusion that the concept of elves was transformed by the Christianisation in two different directions: it was euhemerised in a learned context and demonised in the popular tradition. In the 13th century, the European secular literature which was translated to Old Norse imported new aspects of the concept, aligning it with the image of the European fairies. These three tendencies are represented in different groups of late medieval sources.