Citizen science is a form of collaboration that engages nonprofessionals as contributors to scientific research, typically through the processes of gathering, transforming or analyzing data. To date, research has documented examples of hugely successful citizen science projects, such as Zooniverse and eBird, but citizen science also includes hundreds of smaller citizen science and functionally similar digital humanities projects, operating from small-scale web platforms and in-person collaborative teams. Yet, it is unclear what the "science" of citizen science entails: What are the core research questions and methodologies for answering them? What theories and concepts have been associated with citizen science research to date? What are the technology needs for supporting successful research collaboration among diverse stakeholders and across distinctive types of citizen science projects? In this workshop, our goal is to (i) bring together researchers studying citizen science to form a coherent map summarizing the theories, methodologies and platforms that currently defines citizen science research, with a special focus on CHI and CSCW relevant topics (ii) brainstorm a list of fundamental open questions and ways to tackle them, and (iii) form a multidisciplinary community to build synergies for further collaboration.