On the 11th and 12th February 2015, the Citizen Science 2015 conference took place in San Jose, California. Two busy days of presentations, symposia and workshops provided a great opportunity to learn about and discuss the latest developments in research and practice this field. Citizen science, which has been around for some time as an interesting approach to conduct or support scientific research by including citizens in data collection, analysis or other parts of the research process, is currently going through exciting times. Following the web 2.0 revolution and the new possibilities of organising citizen science projects on the web, participation in citizen science is reaching new levels, and new issues arise, like the managemant of large amounts of data in online portals and data bases, the use of participatory GIS to produce geographic data, and the quality of such data. Clemens Jacobs, member of the GIScience Research Group at Heidelberg University, presented data quality assessment approaches for citizen science observations of plants and animals. Other issues discussed at the conference included educational aspects of citizen science, best practises in organising citizen science projects, or ways of acqiring and keeping participants (also for more sophisticated tasks and activities). A large number of projects and initiatives presented their outcomes and important lessons learned. The conference was also the inaugural event of the newly founded Citizen Science Association (CSA).