The Citizen Science Toolkit is aimed at researchers and scientists working in the fields of biodiversity or environmental sciences who are involved or have an interest in involving citizens in their research projects. The objective of this toolkit is to inform about the potential benefits of citizen science, and to highlight rationales for doing citizen science, current best practices, and useful resources in the field. The toolkit is structured as follows: PART I: INTRODUCTION PART II: BENEFITS & CHALLENGES PART IV: KEY PRINCIPLES & RECOMMENDATIONS FOR SUCCESSFUL CITIZEN SCIENCE PART V: CONCLUSION BIBLIOGRAPHY & RESOURCES