We inhabit space. This means our deepest mental and emotional understanding of the world is tied intimately to our experiences as we perceive them in a physical context. Just like a book or film may induce a sense of presence, so too may our modern sensor drenched infrastructures and mobile information spaces. With the recent development of personal and ubiquitous computing devices that we always carry with us, and increased connectivity and robustness of wireless connections, there is an increasing tie between people and things around them. This also includes the space people inhabit. However, such enhanced experiences are usually limited to a personal environment with a personal smartphone being the central device. We would like to bring such technology enhanced experiences to large public spaces with many occupants where their movement patterns, and interactions can be shared, recorded, and studied in order to improve the occupants' efficiency and satisfaction. Specifically, we use sensor networks and ubiquitous computing to create smart built environments that are seamlessly aware of and responsive to the occupants. Human sensing system is one of the key enabling technologies for smart built environments. We present our research findings related to the design and deployment of an indoor human sensing system in large public built spaces. We use a case study to illustrate the challenges, opportunities, and lessons for the emerging field of human building interaction. We present several fundamental design trade-offs, applications, and performance measures for the case study. / Master of Science / The recent advances in mobile technologies, like smart phones and enhanced wireless communication, allow people to experience added comfort and convenience brought by these devices. For example, smart lighting and air conditioning control can be set remotely, before people arrive at their homes. However, these personal experiences are usually limited to personal spaces and tied to a specific personal smart phone. When it comes to public spaces, we seldom see such technological advancement being utilized. In reality, the concept of smart public spaces is still limited to technologies like opening / closing a door automatically. We discuss the reasons that cause such difference between personal and public spaces. We argue that Human Building Interactions should be shaped around non-intrusive indoor human sensing technologies. We present discussions, considerations and implementation of a system that uses a low cost camera network for indoor human sensing. We also describe several applications based on the developed system. We demonstrate how to bring technology enhanced experiences to public built spaces and provide smart built environments.