To most health economists, “job lock,” the idea that workers work more or face constraints in job mobility due to provision of work-related health insurance, is a real and important phenomenon. It’s one reason why many advocate limiting or ending the exclusion of ESI from taxation, among other reforms. But why, you might ask, do some so firmly believe in job lock? What’s the conceptual or theoretical explanation? Where’s the evidence that it exists and is substantial? If it exists, what laws and regulations help keep it in place? Finally, how does the Affordable Care Act (ACA) begin to address it, if at all?... I will tackle these questions.... I will rely most heavily on the literature reviews contained in Gruber and Madrian (2002), Fairlie, Kapur, and Gates (2011), Bradley, Neumark, and Barkowski (2013), and GAO (2011). Nick’s forthcoming post will cover the legal landscape. He has also provided helpful feedback on early drafts of my posts.