The aim of this thesis is to describe how the picturebook The Cat in the Hat (1957) by Dr. Seuss was adapted into an animated movie (Pratt, 1971) and into a feature film (Welch, 2003) of the same title. An adaptation is a process of moving a story from one medium into another, where many changes occur, including those dependent on the expressive means of the new medium. Dr. Seuss was one of the greatest American children’s authors. His picturebook The Cat in the Hat is considered to be a masterpiece. The animated film adaptation of The Cat in the Hat is an example of a successful adaptation which preserves the integrity of the original story. The feature film adaptation changes the original story of the picturebook by adding new characters, conflicts, events, references, etc. The general meaning and the message of the original is also changed, which has been strongly criticized by the film critics and the general audience. Generally speaking, while adapting a literary work, the authors are expected to respect the original, and, at the same time, to create a new and unique work of art. The analysis of two film adaptations of The Cat in the Hat has shown that the animated film succeeded in meeting these contradictory requirements, while the feature film betrayed the original, and provided a new, but not a very successful piece.