This chapter provides an up-to-date review of the literature on the phonetic and phonological patterns of Welsh and their development. While typically developing children’s acquisition constitutes a major component, it also discusses socio-phonetic variation and adult second language acquisition, thereby approaching Welsh speech development from a lifespan perspective. The chapter is structured in four major sections. The first section introduces the reader to the segmental and suprasegmental properties of the two main varieties of Welsh: Northern and Southern Welsh. Subsequently, the second section considers methodological aspects of studies on Welsh phonology, while the third section focuses on children’s development of Welsh speech patterns, starting with evidence from studies on early word productions before moving on to a discussion of consonant and consonant cluster acquisition in preschool and school-aged children. The section concludes with an account of developmental error patterns. The final major section then reviews the literature on the speech patterns of different groups of Welsh speakers and the role that extra-linguistic variables, such as sex/gender and language dominance, play in shaping these. Finally, studies on the Welsh accents of second language learners will be discussed. The chapter concludes with suggestions for future research.