In this paper, we review the recent literature on assemble-to-order systems. Each assemble-to-order system consists of multiple components and end-products. The components are assembled into the end-products after information on customer demand is received but the decision on what components to procure or produce must be made well before demand materializes. Most of the research in this area aims to find good operating policies to minimize system-wide cost subject to a customer service level. An operating policy of an assemble-to-order system is a combination of a component replenishment/production policy and an inventory allocation policy of these components. The last review on assemble-to-order systems dates back to Song and Zipkin (2003). The literature has been growing quickly in the past few years. Since 2003 more than 100 papers have been published in major operations research and management science journals. We review these papers to serve as a reference for researchers working in the field and as a starting point for those who wish to begin to explore it.