In polysyllabic languages the assignment of stress is crucial for understanding the reading process. Here we review empirical evidence, drawn mainly from studies on Italian, and discuss critical issues in understanding reading. We first discuss the lexical and sublexical mechanisms responsible for stress assignment and propose that the former is based on item-specific knowledge and the latter on the statistical-distributional knowledge that readers have acquired about their language. Then we examine the idea that stress and phonemes pertain to two dimensions of the word, which can be placed at two different representational levels. Finally, we analyze the effects of stress assignment on word articulation, a promising field for future investigation. These issues are addressed by reviewing the studies conducted in adult and young readers to outline the developmental trajectory of stress assignment and discuss how it operates in the reading system.