This book describes three of the main problems that the word-formation process known as conversion presents, namely those related to its definition, its delimitation, and its directionality. The latter constitutes, however, the main focus of the study, which is based on a corpus of over seven hundred lexical units and, more specifically, on 231 actual noun-verb conversion pairs. Considering that directionality is intrinsic to conversion, the main question is whether it is always possible to establish the direction of conversion or whether it is possible to do so only in some cases. Moreover, the study reveals what 'type' of directionality is involved, that is, whether the process is unidirectional, bidirectional or multi-directional. In order to answer these questions, both diachronic (etymology and dates of first records) and synchronic criteria (semantic dependence, restriction of usage, semantic range, semantic pattern, phonetic shape, morphologic type, stress, and the principle of relative markedness) are analysed and assessed. Contents: A problematic word-formation process - The directionality of conversion: a review of previous interpretations - A dia-synchronic study of the directionality of conversion.