The doctorial thesis argues that the term Subcreation with its revised and broadened definition, in part differing from J.R.R. Tolkien's original term sub-creation, may be used for the discussion of the making of fictional worlds in literary discourse. The successful conception of a fictional world depends on the reader's willing suspension of disbelief. This depends both on the author and his skilled composition of the world and all its aspects, as well as on the reader's acceptance of this invented fictional world. The author needs to create a narrative with an inner consistency, which is crucial to achieving the effect of the reader's immersion in the fictional world. The fundamental aspects that an author needs to realize to achieve successful Subcreation have been structured into and analysed in four categories: Language and Linguistic Variation, Physiopoeia, Anthropoeia and Mythopoeia. Furthermore, this thesis shows that, as contemporary examples of fantastic literature, both Tad Williams's and Terry Pratchett's fictional worlds are successfully created through the realization of these aspects of Subcreation. Apart from commenting on the success of the subcreative process, this thesis also remarks upon the cultural influences both authors include in their writings. While both may be considered Anglophone in a general categorization, Pratchett's Discworld retains a feeling of 'Britishness' that is not to be found in Williams's Otherland. The thesis proposes several approaches to Subcreation that may be studied subsequently. So, for example, it may be possible to determine the success of an author's Subcreation by collecting empirical data. Apart from literary works this field of studies may also include other media.