This study focuses on the cognitive processes involved in creole genesis – relexification, reanalysis and direct levelling – processes which the author demonstrates play a significant role in language genesis and change in general. Dr Lefebvre argues that the creators of pidgins/creoles use the parametric values of their native languages in establishing those of the language that they are creating and the semantic principles of their own grammar in concatenating morphemes and words in the new language. This theory is documented on the basis of a uniquely detailed comparison of Haitian creole with its contributing French and West African languages. Summarizing more than twenty years of funded research, the author examines the input of adult, as opposed to child, speakers and resolves the problems in the three main approaches, universalist, superstratist and substratist, which have been central to the recent debate on creole development.