Areal linguistics is having its Golden Age, but its doleful problems as well. Its theory of language contact and linguistic interference is full of controversies which can be resolved only by a thorough analysis of the essentials of language interrelations and the results of their interactions. The monograph is based on a large corpus of linguistic and extralinguistic material collected during the last few decades (including field-work in different countries of Europe, Asia and North America) on the results of language contacts between English, German and other Teutonic languages, between Romance, Slavic and other Indo-European vernaculars, between Uralic, Turkic, Tungusic-Manchurian and Palaeo-Asian, and other languages. An attempt is made to gain an insight into the mechanism of language contact, to single out its types and subtypes and to project the curve of the dynamism of their linguistic interference (the receptor-language levels' penetrability). Special attention is paid to different varieties of plurilingualism.