Cyber-physical systems (CPS), or Internet of Things (IoT) systems, are ubiquitous, encompassing everything from self-driving cars and home appliances to medical and defense systems. The “grand vision” for CPS is an ecosystem where these high-value technologies can be freely composed to grow innovation and the benefits of IoT. This grand vision must cope with two challenges: ease of composition and assurance of trustworthiness, including safety and security. To achieve this vision, we require not only an integrated development environment (to conceive, develop, and test systems) but also a means for composing, monitoring, and assessing key factors like safety in operation. NIST has developed an IoT system framework, a distributed testbed for IoT systems and software that uses the framework’s canonical CPS format to produce dashboards and decision-support reasoning tools. To enable unmanaged composition of IoT systems, they must be made accessible and the system must be accompanied by a “header” that describes how other CPS can accept its outputs. The NIST CPS Testbed is a virtual machine-based cyber and physical co-simulation platform that brings together multi-physics and computational models, including those developed using MathWorks^® tools, to make testing of IoT systems across diverse modeling platforms and remote model locations possible. The CPS framework dashboards and reasoning tools allow decision makers to assess, relative to its release requirements, the status of the system and recommend corrective actions to recover system mission capability and trustworthiness.