This textbook introduces and explains the fundamental issues, major research questions, and current approaches in the study of grammaticalization - the development of new grammatical forms from lexical items, and of further grammatical functions from existing grammatical forms. Grammaticalization has been a vibrant research field in recent years, and has proven effective in explaining a wide range of phenomena; it has even been claimed that the only true language universals are diachronic, and are related to cross-linguistic processes of grammaticalization. The chapters provide a detailed account of the major issues in the field: foundational questions such as directionality, criteria and parameters of grammaticalization, and phases and cycles; the much-debated issue of the motivations behind grammaticalization, including the role of language contact and typological influences; the advantages and disadvantages of different theoretical approaches; and the relationship between grammaticalization and process such as lexicalization, exaptation, and the development of discourse markers. Each chapter offers guidance on further reading, and concludes with study questions to encourage further discussion; there is also a glossary of key terminology in the field. Thanks to its comprehensive approach, the volume will serve as both a textbook for undergraduate and graduate students and a valuable reference work for researchers in the field.