Citizen science is one of the most dramatic developments in science communication in the last generation. But analyses of citizen science, of what it means for science and especially for science communication, have just begun to appear. Articles in this first of two special issues of JCOM address three intertwined concerns in this emerging field: The motivation of citizen science participants, the relationship of citizen science with education, and the implications of participation for creation of democratic engagement in science-linked issues. Ultimately these articles contribute to answering the core question: What does citizen science mean?

Citizen science has proven useful in advancing scientific research, but participant learning outcomes are not often assessed. This case study describes the implementation and tailoring of an in-depth assessment of the educational impact of two citizen science projects in an undergraduate, general education course. Mixed-methods assessment of citizen science within a college classroom demonstrates that public participation in scientific research can positively alter attitudes towards science. The timing and type of assessments yielded significantly different results and qualitative assessment provided depth and context. However, disentangling the impact of the course from participation in the projects is the biggest challenge.

Since 2009 Vetenskap & Allmänhet (Public & Science, VA) coordinates an annual mass experiment as part of ForskarFredag — the Swedish events on the European Researchers' Night. Through the experiments, thousands of Swedish students from preschool to upper secondary school have contributed to the development of scientific knowledge on, for example, the acoustic environment in classrooms, children's and adolescents' perception of hazardous environments and the development of autumn leaves in deciduous trees. The aim is to stimulate scientific literacy and an interest in science while generating scientific output. The essay discusses how the mass experiments can contribute to encouraging scientific citizenship.

Many citizen science projects deal with high attrition rates. The Dutch Great Influenza Survey is an exception to this rule. In the current study, we conducted an online questionnaire (N=1610) to investigate the motivation and learning impact of this loyal, active participant base. Results show that the desire to contribute to a larger (scientific) goal is the most important motivator for all types of participants and that availability of scientific information and data are important for learning. We suggest similar projects seek (social) media attention regularly, linking project findings to current events and including the importance of participants' contribution.

Determined to learn the extent to which a local contaminated site was impacting community health, the Native American community of Akwesasne reached out to a research university, eventually partnering on the first large-scale environmental health community based participatory research project (CBPR). Based on interviews with scientists, community fieldworkers, and study participants, this article examines the ways in which collaborating on these studies was beneficial for all parties — especially in the context of citizen science goals of education and capacity building — as well as the challenges they faced, including communicating the limits of what scientific studies could accomplish for the community.

Open Science may become the next scientific revolution, but still lingers in a pre-paradigmatic phase, characterised by the lack of established definitions and domains. Certainly, Open Science requires a new vision of the way to produce and share scientific knowledge, as well as new skills. Therefore, education plays a crucial role in supporting this cultural change along the path of science. This is the basic principle inspiring the collection of essays published in this issue of JCOM, which deals with many subjects ranging from open access to the public engagement in scientific research, from open data to the social function of preprint servers for the physicians' community. These are issues that go along with the targets of the FOSTER project (Facilitate Open Science Training for European Research) funded by the European Union, which has provided interesting food for thought in order to write this commentary.

The design, delivery and evaluation of JCOM Masterclasses has given us the opportunity to reflect on the audiences, training needs and training schemes available to people working at different levels and in different contexts to communicate STEM subjects to a diverse variety of people. Although not always widely available, short courses in the communication of science have been offered in a number of countries around the world over the past few years. We felt it is now time to open a discussion on the rationale, the methods and the objectives of such training programmes.

For decades, particle physicists have been using open access archives of preprints, i.e. research papers shared before the submission to peer reviewed journals. With the shift to digital archives, this model has proved to be attractive to other disciplines: but can it be exported? In particle physics, archives do not only represent the medium of choice for the circulation of scientific knowledge, but they are central places to build a sense of belonging and to define one's role within the community.

Teaching mathematics in informal settings is a relatively new phenomenon, but it has gained more attention due to the recent changes in the society. The aim of the present quantitative study was to compare the learning outcomes of Latvian and Swedish 12-year-olds when they visited a science centre mathematics-art exhibition originally designed in Estonia. The results showed that in general, prior knowledge of the exhibition contents was the strongest predictor of post-test results in both countries but that mathematical thinking skills and self-concept had a small added value in explaining the post-test results. The results of the study give some of the first pieces of evidence of the effectiveness of out-of-school mathematics teaching in a science exhibition context, providing a good basis for further studies.

Open science is the most recent paradigm shift in the practice of science. However, it is a practice that has emerged relatively recently and as such, its definition is constantly-shifting and evolving. This commentary describes the historical background of open science and its current practice, particularly with reference to its relationship with public engagement with research.