This article deals with the issues of the terminological system formation as a set of professional knowledge terms on the example of English customs activities. The terminology of this fi eld of knowledge plays a key role in the formation of a professional language. It is possible to use diff erent approaches in the study of the problems of systematization and classifi cation of lexical units. Firstly, the composition of the customs dictionary should be considered as a unique, integral structure, including separate components – thematic groups of terms.