Human beings have a genetically-determined capacity to walk, rather than to fly or swim. People can learn to swim, but it is not something that is genetically programmed. Do humans have a genetically-determined capacity to acquire language? Universal Grammar is a theory that assumes that they do. Except in cases of genetic disorder, humans have specialised mental architecture which is uniform across the species in its initial state, and which determines the ways in which samples of language encountered are converted into mental grammars. The specialised architecture is Universal Grammar, and it underlies our capacity to acquire particular languages like English, French, Chinese and so on. Two questions that need to be asked immediately about Universal Grammar if it is to be of any interest in understanding the acquisition of French as a second language are: (i) What evidence is there that Universal Grammar is operating when people who have already acquired a native language learn French as a second language? (ii) What insight does the adoption of a theory of Universal Grammar bring to understanding the processes involved, the course of development over time and the nature of the end state grammars that learners achieve? The article presents empirical evidence from a selection of studies bearing on these questions. It will be argued that the assumption that humans have mental architecture dedicated specifically to language acquisition – Universal Grammar – even in the case of second language acquisition, has allowed considerable progress to be made in understanding second language French.