Two major public health evaluations of e-cigarettes-one from the National Academies of Science, Engineering, and Medicine (NASEM), the other from Public Health England (PHE)-were issued back to back in the winter of 2018. While some have read these analyses as broadly consistent, providing support for the view that e-cigarettes could play a role in smoking harm reduction, in every major respect, they come to very different conclusions about what the evidence suggests in terms of public health policy. How is that possible? The explanation rests in what the 2 reports see as the central challenge posed by e-cigarettes, which helped to determine what counted as evidence. For NASEM, the core question was how to protect nonsmokers from the potential risks of exposure to nicotine and other contaminants or from the risk of smoking combustible cigarettes through renormalization. A precautionary standard was imperative, making evidence that could speak most conclusively to the question of causality paramount. For PHE, the priority was how to reduce the burdens now borne by current smokers, burdens reflected in measurable patterns of morbidity and mortality. With a focus on immediate harms, PHE turned to evidence that was "relevant and meaningful." Thus, competing priorities determined what counted as evidence when it came to the impact of e-cigarettes on current smokers, nonsmoking bystanders, and children and adolescents. A new clinical trial demonstrating the efficacy of e-cigarettes as a cessation tool makes understanding how values and framing shape core questions and conclusive evidence imperative.