In this dissertation, I explore how the knowledge of people's and devices' spatial relationships -called proxemics -can be applied to the design of ubiquitous computing (ubicomp) interactions. Edward Hall's proxemics theory describes how people use spatial relationships -such as varying their distance or orientation -to mediate their interactions with other people around them. But in spite of the opportunities presented by people's natural understanding of proxemics, only a relatively small number of ubicomp installations incorporate proxemic information within interaction design.Therefore, my goal in this dissertation research is to inform the design of future proxemic-aware devices that -similar to people's natural expectations and use of proxemics -allow increasing connectivity and interaction possibilities when in proximity to people, other devices, or objects. Towards this goal, I explore how the finegrained knowledge of proxemic relationships between the entities in small-space ubicomp ecologies can be exploited in interaction design. In particular, I provide the following three major contributions: First, I operationalize proxemics for ubicomp interaction with the Proxemic Interactions framework that serves to guide the design of ubicomp applications. The framework describes how designers can consider fine-grained proxemic information to mediate people's interactions with digital devices, such as large digital surfaces or portable personal devices. I identify five key dimensions of proxemic measures (distance, orientation, movement, identity, and location) to consider when designing proxemicaware ubicomp systems. I also identify the gradual engagement design pattern as one particular strategy that allows designing system interactions that move from awareness, to reveal, to interaction.-iiiSecond, I design the Proximity Toolkit allowing ubicomp developers to rapidly prototype proxemic-aware ubicomp systems. The toolkit simplifies the development process by supplying higher-level information about proxemic relationships between the entities in ubicomp ecologies through an event-driven API and visual inspection tools.Third, I explore the design of three case studies of proxemic-aware systems that react continuously to people's and devices' proxemic relationships. The case studies explore the application of proxemics in small-space ubicomp ecologies by considering first person-to-device, then device-to-device, and finally person-to-person & device-to-device proxemic relationships. Together, they validate the toolkit's versatility and the application of the Proxemic Interactions framework.-iv -