English is a free-stress language, meaning that word stress can fall on various syllables, depending on the word. Stress also affects the way that vowels and consonants are pronounced, and influences the ways that listeners identify the words being spoken. Because of this, misplaced word stress in English can stop communication completely. When a word cannot be recognized, listeners may be slowed down or stop all other processing to decode the word that was not understood. The stressed syllable of words is particularly important in word recognition because it can lead listeners to look for words that are quite unrelated to the one that was spoken. Because listeners use segmentals and stress patterns to identify possible words before a word is fully articulated, wrong stresses may lead listeners to look for incorrect words, compromising intelligibility and/or comprehensibility. This chapter argues that word stress is essential in an intelligibility-based approach to teaching pronunciation. The chapter also argues that word stress is important for ESL and ELF contexts because of the impossibility of separating stress and segmentals, evidence that stress affects both native and nonnative listeners, and findings that show how misplaced word stress can severely impact intelligibility for various learners.