Addressing the question of what makes an online social system “viable” requires some preliminary conceptual clarifications in order to define the scope of the present analysis. Section 4.1 of the present chapter is devoted to framing the problem conceptually: we first introduce the notion of a collaborative Web community by considering the properties that characterise it; we then discuss a number of ways in which the viability of these systems can be defined and the challenges faced by empirical research in identifying measurable indicators of viability. In Sect. 4.2 we present an empirical analysis of two paradigmatic cases of collaborative Web communities and discuss methodological issues emerging from the study of their dynamics from the point of view of viability. We conclude by presenting in Sect. 4.3 a simple model of viable online communities based on the empirical and conceptual considerations of the first two sections.