Fiber-reinforced polymer composites have gained popularity through their advantages over conventional metallic materials. Most polymer composites are traditionally made with reinforcing fibers such as carbon or glass. However, there has been recent interest in sourcing these reinforcing fibers from renewable, natural resources. Nanocellulose-based reinforcements constitute a new class of these naturally sourced reinforcements. Some unique behavior of nanocellulose creates both opportunities and challenges. This chapter reviews the progress and some of the remaining issues related to the materials, processing, and performance of nanocellulose reinforced thermosetting composites.