For more than 20 years, scholars have used the term “attitude strength” to refer to the durability and impactfulness of attitudes, and a large literature attests to the important leverage that this concept offers for understanding and predicting behaviour. Despite its prominence, however, a number of fundamental questions remain regarding the structure and function of attitude strength. In this chapter we draw on a wide range of evidence to clarify the nature of attitude strength. Rather than conceiving of attitude strength as a meaningful psychological construct, we argue that it is better conceptualised as an umbrella term that refers in only the most general way to multiple, separable classes of attitude outcomes, instigated by different antecedents and produced by distinct psychological processes. Although strong attitudes share a set of general qualities—resistance to change, persistence over time, impact on thought and behaviour—there are many distinct routes by which attitudes come to possess these qualities, and many diverse ways in which these qualities manifest themselves. Our analysis shifts the focus away from the structural properties of attitude strength and towards a fuller appreciation of the distinct sources from which attitudes derive their strength. We argue in particular for the value of attending more closely to the social bases of attitude strength, and we illustrate the value of this approach by reviewing several lines of research.