Language identification (LI) is the task of predicting the language(s) in a text or speech input. The main difference between LI of text and speech is that the characters that make up the text are discrete, whereas with speech, the input is usually a continuous signal. This means that different styles of mathematical methods are needed to process text and speech, traditionally with little methodological overlap between them. In this book, we focus on the language identification of digital text, although we do touch on applications to speech in the case that the speech signal has been translated into a sequence of (discrete) phones. Recognizing the language(s) that a text is written in comes naturally to a human reader familiar with the language(s). Table 1.1 presents excerpts from Wikipedia articles in four different European languages on the topic of Natural Language Processing (NLP), labeled according to the language they are written in. Without referring to the labels, readers of this book will certainly recognize at least one language, and many are likely to identify all of them, even if they can’t read the content in all cases.