Occupant monitoring is an important part of various smart infrastructure systems such as smart healthcare, security, and efficient energy management. The application of current sensing approaches for occupant monitoring is limited in real life scenarios due to sensing requirements such as line of sight, dense deployment, and the need to carry a device. To reduce these sensing requirements, Mostafa and his team has introduced Structure as Sensors (SaS) for characterizing and monitoring occupants through their footstep-induced floor vibration. Conventionally, this characterization occurs through inverse data-driven modeling which utilizes the vibration responses to extract information about the excitation source. However, due to the structure-dependent nature of these responses, the data-driven approach requires significant amount of labelled data in every structure which is impractical. In this talk, Mostafa presented a physics-guided model transfer approach which characterizes the effect of the structure on the footstep-induced floor vibration responses to enable footstep modeling across structures. Further, he discussed their evaluation in multiple commercial and residential buildings which has been performed through collaboration with various nursing homes and hospitals for monitoring the patients and elderly population. Finally, Mostafa mentioned some of the potential applications of SaS in other domains of civil engineering.