Executive functions (EF) are domain general cognitive processes that contribute to individual learning, problem solving, and planning across the lifespan. Multidisciplinary interest in EF skills stems from their broad relevance to functionally important outcomes. However, this multidisciplinary interest in EF has also resulted in a large literature that is not internally consistent. In this chapter, we characterize historical shifts in the conceptualization and definition of EF skills. We describe quantitative and qualitative aspects of developmental change in EF skills. Finally, we highlight discrepancies that exist between current measurement approaches. The inconsistencies and discrepancies that we highlight are not surprising given the maturity of the EF literature. By drawing attention to these issues, we aim to provide readers a broad vantage from which they can critically evaluate the other chapters in this volume.