This chapter is about citizen science: scientific work undertaken by members of the general public, often in collaboration with or under the direction of professional scientists and scientific institutions. In this chapter, citizen science is defined, and the development is briefly described. The number of projects involving citizen science has increased significantly recently, and data collection of this type is often used for monitoring. The reasons for using citizen science as a methodological approach are diverse, but transferring knowledge to the society is always important. However, knowing how to handle and train participants is important if the project is to be successful, and all projects need to allocate additional resources for quality control and ethical considerations. This chapter discusses new technology and considerations around the use of volunteers in monitoring and provides examples of the use of citizen science in a range of different monitoring approaches.