Although attachment is a long-established area of psychological research, it has been characterised by major controversies throughout most of its history. Indeed, authors from different theoretical orientations find it difficult even to agree on a definition of attachment! Given this state of affairs, it is appropriate that the definition of attachment is an early issue discussed in this chapter. Having considered this fundamental question, the chapter then provides a basic introduction to attachment theory and research. Fundamental questions are addressed, such as how do children become attached to their parents? How can we measure attachment? And, how do patterns of attachment vary from one child to another? These are important questions but even more significant is the potential impact of attachment on children's subsequent relationships. The final section of the chapter focuses on the impact of early attachment on later peer relationships throughout childhood and adolescence.