In the last two sections of this part of the book, we examine some of the developmental and psycholinguistic issues that arise in connection with sound systems. Section 6 discusses how phonology can throw light on the acquisition of pronunciation patterns by children learning their first language. Additionally, it illustrates the interaction between approaches alluded to in the main introduction, in that we will see that aspects of child phonology require theoretical notions which also find a role in the formulation of adult grammars. Finally, in section 7, we will consider selected aspects of speech perception, along with common everyday errors in speech production (so-called slips of the tongue). This section concludes with a brief discussion of the role of phonology in understanding certain aspects of poetic systems and the way that writing systems have developed. Overall, the section seeks to establish the importance of some of the theoretical notions introduced in section 5 for the understanding of phenomena with which some readers will already be familiar.