English is an Indo-European language and belongs to the West Germanic group of the Germanic languages. Old English originated from a Germanic tribal and linguistic continuum along the Frisian North Sea coast, whose languages gradually evolved into the Anglic languages in the British Isles, and into the Frisian languages and Low German/Low Saxon on the continent. The Frisian languages, which together with the Anglic languages form the Anglo-Frisian languages, are the closest living relatives of English. Low German/Low Saxon is also closely related, and sometimes English, the Frisian languages, and Low German are grouped together as the Ingvaeonic (North Sea Germanic) languages, though this grouping remains debated. Old English evolved into Middle English, which in turn evolved into Modern English. Particular dialects of Old and Middle English also developed into a number of other Anglic languages, including Scots and the extinct Fingallian and Forth and Bargy (Yola) dialects of Ireland.

Like Icelandic and Faroese, the development of English in the British Isles isolated it from the continental Germanic languages and influences. It has since evolved considerably. English is not mutually intelligible with any continental Germanic language, differing in vocabulary, syntax, and phonology, although some of these, such as Dutch or Frisian, do show strong affinities with English, especially with its earlier stages.

Unlike Icelandic and Faroese, which were isolated, the development of English was influenced by a long series of invasions of the British Isles by other peoples and languages, particularly Old Norse and Norman French. These left a profound mark of their own on the language, so that English shows some similarities in vocabulary and grammar with many languages outside its linguistic clades—but it is not mutually intelligible with any of those languages either. Some scholars have argued that English can be considered a mixed language or a creole—a theory called the Middle English creole hypothesis. Although the great influence of these languages on the vocabulary and grammar of Modern English is widely acknowledged, most specialists in language contact do not consider English to be a true mixed language.

English is classified as a Germanic language because it shares innovations with other Germanic languages such as Dutch, German, and Swedish. These shared innovations show that the languages have descended from a single common ancestor called Proto-Germanic. Some shared features of Germanic languages include the division of verbs into strong and weak classes, the use of modal verbs, and the sound changes affecting Proto-Indo-European consonants, known as Grimm's and Verner's laws. English is classified as an Anglo-Frisian language because Frisian and English share other features, such as the palatalisation of consonants that were velar consonants in Proto-Germanic (see Phonological history of Old English § Palatalization).