This thesis examines how the differences between Tolkien's novel - the Lord of the Rings - and its filmic adaptation can be explained. In particular, how the changes can be related to the different nature, conventions and genres of the two media. The main interest areas are characterisation, temporality and causality, and language, humour and verisimilitude. Two facets of the adaptation process that are very important to the discussion are mainstreaming and generic translation, as they can explain so many of the changes and similarities as well. This thesis refrains from fidelity criticism and tries to examine the novel as a novel and the films as films to the degree this is possible.