This chapter introduces Mora Loss and Restoration (MLR) Theory, which posits that vowel reduction always involves mora loss. In this process, unstressed vowels lose their moras and function as non-moraic syllable heads. As a result, high and low vowels (e.g. /i, u, a/) become their corresponding glides, while mid vowels (e.g. /e, o/) either centralize to central glides or raise to high glides, depending on the grammatical system. In certain prosodic contexts, unstressed vowels may undergo mora restoration, regaining moraic status. This process is observed in New Zealand English and European Portuguese speakers. Beyond mora loss and mora restoration, MLR identifies various feature change processes that interact with and are influenced by these moraic shifts. One key process is feature loss, where unstressed vowels reduce to schwa. The chapter also explores the implementation of MLR Theory within both rule-based and constraint-based frameworks. Finally, the chapter discusses the phonetic and phonological evidence required to support MLR Theory, highlighting the importance of experimental studies for its validation.