Rhythm, and its related connected speech modifications, were once central to pronunciation teaching, but more recently have been relegated to the sidelines, especially in English as a lingua franca. This chapter argues that rhythm is central to intelligibility in production and perception. In regard to perception, the chapter agrees with almost every other approach. L2 learners need to be able to understand the speech of others, especially native speakers, whose stress-based rhythm is often challenging. In regard to production, research on lexical segmentation (the identification of words in connected speech) has shown that all listeners, native as well as nonnative, struggle to segment speech (i.e., they struggle with intelligibility) when they have to process speech with unfamiliar rhythm. Rhythmic differences can cause difficulties for native listeners but also for nonnative listeners. Rhythm underlies how listeners process speech and segment the stream of speech into identifiable words and messages, both key elements of intelligibility. This indicates that rhythm is critical for intelligibility, and affects how listeners access words and messages. Different rhythmic structures across languages guarantee that rhythm will affect how listeners and speaker understand each other.