This chapter explores what authors mean by networks and provides an initial outline of the factors that influence their success. A growing number of school leaders and policy-makers are increasingly turning their attention to Professional Learning Networks (PLN) as a way of improving education in schools and across school systems. This focus is also reflected in academia, where educational researchers, such as Hargreaves Andy and Stoll, argue that learning networks are fundamental to achieving effective school improvement. PLNs also provide the opportunity to achieve cost-effective educational change at scale. The implications of this focus are clear: there is now a real need to ensure teachers participate in learning network activity as well as share the learning that results from this activity with others. The chapter also presents an overview of the key concepts discussed in this book.