Education-job mismatch--a signal of market failure due to the inefficient assignment of workers to positions--is one of the main features of the modern labor market (Leuven & Oosterbeek; 2011; Quinitini; 2011; Verhaest & Van der Velden; 2013). Mismatch refers to over- and undereducation on the job market when workers hold educational qualifications above or below those required for their current jobs. Studies that examine education-job mismatch conventionally rely on theories of human capital, signaling theory, search and match theory, although in general, these theories were not originally developed to account for immigrants' labor market experiences. The goal of this study is to empirically assess the extent of education-job mismatch and identify factors that exacerbate or mitigate mismatch within the population of U.S. employed workers between the ages of 20 to 65. The study restricted the analysis to this age bracket because it was expected workers between these ages to be in the labor force full-time and the goal was to exclude workers that might be employed part-time because of school or retirement. The study examines differences in the over and undereducation between workers by immigrant generation, race, gender, and worker characteristics, including the extent to which cognitive skills measured by the Program for the International Assessment of Adult Competencies (PIAAC) numeracy score mediate mismatch.