This chapter is dedicated to sensing communities, which I describe as groups of people who collectively engage in collecting, interpreting, and communicating evidence to address environmental or social issues. Their social ties and shared practices are closely intertwined with the purpose of their activities of data collection and advocacy. This form of digital community gained prominence around the early 2010s, a period marked by widespread optimism about the power of digital platforms to engage and mobilize for collective action. The Arab Spring had just unfolded, toppling governments across the Middle East and North Africa. Activists used Facebook and Twitter to coordinate protests, disseminate information while circumventing censorship, fueling an emerging democratic movement. The crisis mapping movement emerged around the same time as a prominent collaborative practice. Social media users across the globe collected, sorted, and organized geolocated messages such as tweets and SMS to support humanitarian efforts.