The NIST-organized Lightweight Cryptography Workshop 2015 will be held on July 20-21, 2015, in Gaithersburg, Maryland. There are several emerging areas in which highly constrained devices are interconnected, typically communicating wirelessly with one another, working in concert to accomplish some task. Examples of these areas include: sensor networks, healthcare, distributed control systems, the Internet of Things, cyber-physical systems, and the smart grid. Security and privacy can be very important in all of these areas. Because the majority of modern cryptographic algorithms were designed for desktop/server environments, many of these algorithms cannot be implemented in the devices used by these applications. When current algorithms can be engineered to fit into the limited resources of constrained environments, their performance is typically not acceptable. NIST seeks to discuss issues related to the security and resource requirements of applications in constrained environments, and potential future standardization of lightweight primitives.