"The Kauffman Index: Main Street Entrepreneurship" is a novel indicator of small business activity in the United States, integrating high-quality sources of timely information into one composite indicator. The Index captures business activity in all industries, and is based on both a nationally representative sample size of roughly 900,000 responses each year and on the universe of all employer businesses in the United States. The focus here is on established small businesses—employer firms older than five years and with fewer than fifty employees—and the business owners based on a location. As such, we examine both the business owners and the businesses they own. Main Street entrepreneurship is an important aspect of the U.S. economy and society. Established small businesses make up 63 percent of all employer firms in the United States1 and are a source of local economic activity. Entrepreneurship is a multi-dimensional phenomenon, and the Kauffman Index of Entrepreneurship series—the umbrella initiative for all Kauffman Index reports—studies and analyzes entrepreneurship from various perspectives. Another report, the Kauffman Index: Startup Activity, seeks to view the beginnings of entrepreneurship, focusing on new business creation, market opportunity, and startup density. This series, the Kauffman Index: Main Street Entrepreneurship, takes a different angle and attempts to understand prevalence of local small business. This report presents trends in Main Street entrepreneurship activity for the forty largest metropolitan areas in the United States by population. Nationwide Main Street entrepreneurship activity—an indicator of the number of established small businesses and the number of business owners in a location— experienced a large increase in 2015, reversing a six-year downward and stagnant trend in the United States.