Online social networks have made sharing personal experiences with others mostly in form of photos and comments a common activity. The convergenceof social, mobile, cloud and wearable computing expanded the scope of usergeneratedand shared content on the net from personal media to individual preferencesto physiological details (e.g., in the form of daily workouts) to informationabout real-world possessions (e.g., apartments, cars). Once everydaythings become increasingly networked (i.e., the Internet of Things), future onlineservices and connected devices will only expand the set of things to share. Given that a new generation of sharing services is about to emerge, it is of crucialimportance to provide service designers with the right insights to adequatelysupport novel sharing practices. This work explores these practices within twoemergent sharing domains: (1) personal activity tracking and (2) sharing economyservices. The goal of this dissertation is to understand current practices ofsharing personal digital and physical possessions, and to uncover correspondingend-user needs and concerns across novel sharing practices, in order to map thedesign space to support emergent and future sharing needs. We address this goalby adopting two research strategies, one using a bottom-up approach, the otherfollowing a top-down approach.In the bottom-up approach, we examine in-depth novel sharing practices within two emergent sharing domains through a set of empirical qualitative studies.We offer a rich and descriptive account of peoples sharing routines and characterizethe specific role of interactive technologies that support or inhibit sharingin those domains. We then design, develop, and deploy several technology prototypesthat afford digital and physical sharing with the view to informing the design of future sharing services and tools within two domains, personal activitytracking and sharing economy services.In the top-down approach, drawing on scholarship in human-computer interaction (HCI) and interaction design, we systematically examine prior workon current technology-mediated sharing practices and identify a set of commonalitiesand differences among sharing digital and physical artifacts. Based uponthese findings, we further argue that many challenges and issues that are presentin digital online sharing are also highly relevant for the physical sharing in thecontext of the sharing economy, especially when the shared physical objects havedigital representations and are mediated by an online platform. To account forthese particularities, we develop and field-test an action-driven toolkit for designpractitioners to both support the creation of future sharing economy platformsand services, as well as to improve the user experience of existing services.This dissertation should be of particular interest to HCI and interaction designresearchers who are critically exploring technology-mediated sharing practicesthrough fieldwork studies, as well to design practitioners who are building and evaluating sharing economy services.