This chapter is about the question of what creates and sustains viability in Communities of Practice (CoPs) embedded in an organizational context. Experience with successful CoPs at Siemens AG has shown that even though most of them differ greatly from each other in many aspects, they all share five common factors that are necessary for the viability of a CoP. These five factors are introduced in the following pages. They represent an approach that can be used to analyse and improve CoPs that do not seem to be viable and as a guide for CoP members and moderators to maintain viability in their own CoPs.