Vowel and consonant sounds are the heart of traditional pronunciation teaching, and they remain essential in teaching for intelligibility. One simply cannot speak clearly without sufficient accuracy in pronouncing segmentals. This is true for perception as well as production. This chapter describes segmentals from various perspectives, showing how they are distinct from, yet overlapping with, the suprasegmentals of the language. All segmentals are unavoidable in speaking, and how they are pronounced influences how well listeners understand speakers. This chapter also looks at segmentals from the perspective of L2 intelligibility and argues that they are most likely to affect the intelligibility of words. It describes how different errors affect intelligibility. First, segmentals are critical in judgments of intelligibility and comprehensibility. Second, some segmentals are more likely to cause misunderstandings than others, and different types of mispronunciations are likely to have varied effects on intelligibility, especially if they are combined with other errors. Finally, the chapter describes what is known about both naturalistic development of segmentals and the effects of instruction