Weightless Segments deals with a specific question that arises when we regard the stress rules of quantity-sensitive languages. In these languages, vowels and coda consonants can, by virtue of their presence or absence, influence the weight of the syllable, and thus, indirectly, the location of word stress. It is common knowledge that onset consonants do not have this capacity. The question that this book, researched within the laboratory phonology framework, tries to answer is why. A phonetic explanation for the weightless behaviour of syllable onsets is sought in the durational behaviour of onset, nucleus and coda. Assuming that duration is the primary phonetic correlate of phonological weight, an asymmetry in the durational behaviour of these subsyllabic constituents might explain the observed differences in potential weight. A series of production and perception experiments was conducted to reveal this asymmetry, and to explain it. A further task that is undertaken in this book is the reanalysis of a group of, mostly Australian Aboriginal, languages for which some degree of onset influence in their stress rules has been claimed in the past (in defiance of the universal rule that onsets do not count). In the second part of this book, after a general overview of stress in Aboriginal languages, it is shown that these offending languages can all be reanalysed without any reference to onset influence whatsoever.