This study benefited from the contributions of many individuals and organizations. The authors wish to thank the many staff at the Internal Revenue Service who provided information on current IRS activities and the likely impact of new authentication mechanisms. The study also would not have been possible without the thoughtful contributions of identity providers and cyber security experts whose participation, while confidential, offered the necessary insight into how the identity ecosystem is expected to function under the National Strategy for Trusted Identities in Cyber Space. Lastly, the authors wish to thank several individuals in particular for their comments on draft material and their insights and recommendations over the course of the study. Many thanks to Jeremy Grant (NIST), Mike Garcia (NIST), Rich Phillips (IRS), Gregory Tassey (NIST), and Gary Anderson (NIST).