Modern Basque dialects show a great deal of diversity in accentual or stress systems. In this chapter, an overview of these systems is presented, with special focus on accented morphemes, and clues to the reconstruction of the Proto-Basque stress system. Earlier reconstructions of Proto-Basque accent are reviewed, and the extensive work of Hualde on this topic is summarized. Based on previous descriptions, a quantity-sensitive stress system is proposed for Proto-Basque where heavy syllables attract stress so that CVCVC stems will have final stress, while CVCV and CVCCV stems will show initial stress. With the stress system in place, a number of sound changes can be shown to be sensitive to prosodic structure, including *h-loss in initial unstressed syllables, *rh > rr before unstressed vowels, and vowel reduction and loss in compounds. The chapter concludes with a summary of processes leading to more and more iambic structures in the language, from morphological prefixation of CV- syllables, to CVC > CV reduction in reduplicated monosyllables.