In the context of English as a global language in which the interaction is not only between native speakers and English as second language speakers but also among non-native speakers themselves. This trend has resulted in and increasing the need to acquire certain knowledge, creative abilities for intelligible communication with different English speakers. The intelligibility of English speakers is affected by various factors, so to achieve successful communication, there is a need to investigate what linguistic features affect the intelligibility of English speakers especially to those who use English as a foreign language, and how foreign-accented English is judged by different groups of listeners. This book addresses these issues on the particular group of Vietnamese speakers and various groups of English listeners. Implications in English language teaching are drawn by the authors from the findings.