The many characteristics that make the IoT different from traditional networked computing also require us to pay particular attention to how to make such systems safe and secure. This Building Code document focuses on the challenges associated with composing systems, rather than building individual programs or devices. We use the concept of smart cities to illustrate how design for safety, security, and privacy must consider emergent properties, and how a system or technology designed for this domain must account for how it might be integrated, reused, or composed with other technologies and systems.