Combining cross-linguistic typology, experimental data and formal analysis, this book introduces a new theoretical model for understanding how and why vowels change in unstressed syllables - Mora Loss and Restoration (MLR) Theory. In MLR Theory, unstressed vowels lose moras – phonological elements that represent duration. This loss, which is distinct from feature loss, has pervasive phonological and phonetic effects, but can be reversed later in the derivation. This book addresses methodological challenges, emphasizing the importance of morphophonological alternations and acoustic measurements, and offers a comprehensive typology of vowel reduction patterns. The theory is backed up with a wealth of data from New Zealand English and European Portuguese speakers, bridging abstract phonological theory with concrete evidence. Written for researchers and students of phonology, phonetics and morphology, this book is a valuable resource for those exploring the theoretical and empirical dimensions of vowel reduction across languages, and especially the interaction of prosody and segments.