Language contact is an important factor in historical language development. When speakers of different languages interact, they may borrow words, expressions, sounds, or grammatical patterns from one another. Trade, migration, colonization, education, travel, and political relationships can all contribute to language contact. Borrowing demonstrates that language change is not always an internal process; languages can also be transformed through interaction with other languages.

Another important concept is grammaticalization, the historical process through which lexical words or independent constructions develop grammatical functions. A word that originally has a concrete lexical meaning may gradually become an auxiliary, grammatical marker, or other functional element. Grammaticalization is important because it shows how grammar can emerge and develop through repeated patterns of language use.

Comparative linguistics contributes significantly to historical language study by comparing related languages. Linguists look for regular similarities in sounds, words, and grammatical structures to determine historical relationships. This approach has helped linguists classify languages into families such as Indo-European, Sino-Tibetan, Austronesian, and Afro-Asiatic. Comparative analysis can also provide evidence for reconstructing forms that existed in ancestral languages.

Etymology is the study of the origins and historical development of individual words. It traces words through earlier stages and identifies their sources. For instance, many English words can be traced to Latin and French as a result of historical contact.
Etymological analysis can therefore reveal not only the history of a word but also aspects of cultural interaction between language communities.

Language reconstruction is another important area of historical linguistics. When an ancestral language has not been directly recorded, linguists can use evidence from related languages to reconstruct possible earlier forms. The comparative method is particularly useful for this purpose. Reconstruction does not necessarily produce an exact record of the ancestral language, but it provides hypotheses based on systematic linguistic evidence.

Historical dialectology examines how regional and social varieties of a language develop over time. Dialects may become different because of geographical separation, migration, social identity, political boundaries, and interaction with other speech communities. Historical dialectology is particularly useful for understanding how a single language can develop into several distinct regional varieties.
Historical linguistics also considers the causes of language change. Linguistic changes may result from internal factors such as ease of pronunciation, analogy, and grammatical restructuring. External factors include language contact, migration, social change, political events, technological developments, and cultural influence. In many cases, several factors operate together rather than a single factor causing a particular change.

The relationship between synchronic and diachronic linguistics is also important. A synchronic study provides a description of the linguistic system at a particular stage, while a diachronic study connects different stages and explains how one state develops into another. For example, understanding the present-day English verb system requires synchronic description, but understanding why the system has its current form requires diachronic investigation. Thus, the two approaches complement rather than contradict each other.