本文从历史的视角讨论应用语言学思想的缘起、变化和发展：从广义应用语言学与狭义应用语言学之分，到应用语言学的支撑学科与子学科的分别，再到应用语言学的研究范围不断延伸，从而导致了G. Brown、H. Widdowson、M. Halliday三种不同的思想。最后文章论述了二语习得作为应用语言学姊妹学科的出现及其相互关系。

This paper traces the origin, changes and development of the conception of Applied Linguistics from a historical perspective, from the division between macro and micro Applied Linguistics to that between the subfields and supporting disciplines, and finally to the broadening of the scope of applied linguistic research, leading to three different approaches to it developed by G. Brown, H. Widdowson and M.A.K. Halliday. Second Language Acquisition then emerged as its sister discipline.