In recent years, the concept of evolvability has been gaining in prominence both within evolutionary developmental biology (evo-devo) and the broader field of evolutionary biology. Despite this, there remains considerable disagreement about what evolvability is. This article offers a solution to this problem. I argue that, in focusing too closely on the role played by evolvability as an explanandum in evo-devo, existing philosophical attempts to clarify the evolvability concept have been overly narrow. Within evolutionary biology more broadly, evolvability offers a (...) robust explanation for the evolutionary trajectories of populations. Evolvability is an abstract, robust, dispositional property of populations, which captures the joint causal influence of their internal features on the outcomes of evolution (as opposed to the causal influence of selection, which is often characterized as external). When considering the nature of the physical basis of this disposition, it becomes clear that the many existing definitions of evolvability at play within evo-devo should be understood as capturing only aspects of a much broader phenomenon. 1 Introduction2 The Problem of Evolvability3 The Theoretical Role of Evolvability in Evolutionary Biology 3.1 The explanatory targets of evolutionary biology 3.2 Selection-based explanations 3.3 Lineage explanations 3.4 Evolvability-based explanations 3.5 What properties must evolvability have?4 What Evolvability Really Is 4.1 Making sense of ft 4.2 Making sense of x and b5 What of the Limbs? The Power of E6 Conclusion. (shrink)