This chapter explains the insights and techniques of historical linguistics, the study of how language changes over time. We begin with a brief explanation of the value of historical linguistics before going on in Section 10.2 to describe the background to its development. In Section 10.3 we examine some key explanations for why languages change over time. In so doing we discuss how changes come about and how they then spread throughout speech communities. Following this, we describe some of the types of change that languages go through. Here we focus particularly on change at the levels of phonology, grammar, lexis and semantics. We give examples of changes that have occurred at these levels of language and describe some influential theories that have sought to explain such developments. We then move on in Section 10.5 to describe some of the main techniques that historical linguists use to study language change. By the end of this chapter you should have a good knowledge of the variety of ways in which languages change, as well as an insight into the methods that linguists use to study such changes. INTRODUCTION Primarily, historical linguistics involves describing how and explaining why language changes over time. Much work has focused on sound change, such as the fact that Germanic languages, past and current, have the sound [t] at the beginning of the number ‘two’ (e.g. Gothic twai, English two, Dutch twee, German zwei (pronounced [tsvai])), whereas other Indo-European languages past and current typically have [d] (e.g. Latin duos, Italian due, French deux, Spanish dos, etc.). But other linguistic areas - grammar, semantics and lexis - have also received significant attention, and most recently areas such as pragmatics have come into focus. Why bother with historical linguistics? Importantly, it helps explain language that is used today. For example, why is it that when we say the English word knight we do not pronounce the <k>? Historical linguistic detective work has been able to establish that the <k> used to be pronounced. But that sound has been subject to a regular process of sound change, mapped out by historical linguists, until it reached the final endpoint of complete loss. Today’s spelling simply retains the <k> as an archaeological relic.