The thesis of the paper is that the process of learning a second language, if successful, is the same as that of learning a first one. The paper discusses various objections that have been raised against this thesis, and it discusses the considerable body of research which explores it. It examines the appropriateness of the research data for throwing light on the validity of the thesis. It concludes with some practical guidelines for language teachers drawn from observations of babies learning their mother tongue. (Author)