The concept of the 'onset', i.e. the consonant(s) before the vowel of a syllable, is critical within phonology. While phonologists have examined the segmental behaviour of onsets, their prosodic status has instead been largely overlooked. In fact, most previous accounts have stipulated that onsets are insignificant when it comes to the 'heaviness' of syllables. In this book Nina Topintzi presents a new theory of onsets, arguing for their fundamental role in the structure of language both in the underlying and surface representation, unlike previous assumptions. To capture the weight behaviour of onsets, a novel account is proposed that relates their interaction with voicing, tone and stress. Using numerous case-studies and data from a variety of languages and phenomena (including stress, compensatory lengthening, gemination and word minimality), the book introduces a model that reflects the true behaviour of onsets, demonstrating profound implications for syllable and weight theories.