Contributing to a growing attention to algorithms and algorithmic interaction in the CHI and CSCW communities, this workshop aims to deal centrally with the topic of human “participation” and its changing role to data-driven, algorithmic ecosystems. Such a focus includes projects that involve users in the design of algorithms and “human-in-the-loop” systems, broader investigations into the ways in which “participation” is situated in data-driven activities, as well as conceptual concerns about participation’s changing contours in contemporary social computing landscapes. This one-day workshop led by academic and industry researchers and sets out to achieve three goals: identify cases and ongoing projects on the topic of participation in algorithmic ecosystems; create a tactical toolkit of key challenges and strategies in this space; and set a forward-facing agenda to provoke further attention to the changing role of participation in contemporary sociotechnical systems.