This chapter provides an introductory overview of the phonological systems used in the languages of mainland Southeast Asia. The chapter begins with an overview of the various systems of consonants and vowels that are found, and the phonotactic principles by which those segments are combined to form syllables. The closely-related phenomena of register and tone are then introduced and defined, followed by an outline of the processes of tonogenesis, by which a language can develop a tone system. The chapter closes with a discussion of further aspects of the area’s sound systems, including tone sandhi, intonation, the prosodic hierarchy and criteria for different notions of ‘word’.