The U.S. retail sector has changed over the past three decades from one with many small firms to one dominated by large firms. The first chapter uses new data to document these changes at both the national and local level. It shows that concentration has been increasing in the U.S. retail sector at both the local and national level for the past 30 years. It shows that the primary contributor to the rise in national retailers is the growth of national retail firms that have stores in many markets and the exit of small retail firms. The second chapter of this work relates those changes to trade by establishing that stores of small retail firms are more likely to close when their competitors import directly. This work combines detailed measures of store sales with data on imports of each store's competitors to establish that stores of small firms are more likely to close when their competitors import directly. The final chapter of this work estimates a model of retailer entry decisions with direct imports to estimate the net effect of imports on local retail markets. I estimate that shutting down trade would not decrease local concentration because although small retail firms would enter markets, the largest retailers would exit. Combined these papers indicate that trade may have lead to some increases in national retail concentration, but cannot explain the increases in local retail concentration.