New citizen science projects are emerging all the time as scientists, policy-makers, and non-governmental organisations see the value of conducting research in this way. Understanding what factors influence people to take part in citizen science projects and why participants continue their involvement are important questions for the field. Here, we bring together key theories from the volunteering literature with examples from the environmental volunteering and citizen science literature to describe the factors that influence people to start and continue participating in citizen science projects. Good project organisation is key, and project organisers need to consider potential participants’ motivations; their personal attributes, circumstances and demographics; and how they will become aware of the opportunity. We discuss each of these factors with reference to both the academic and the grey (non-academic) literature, and based on this make general recommendations for those designing and running citizen science projects.