This chapter focuses on how, by “activating” the citizen's engagement in the research process, the scientific community has a smart way to benefit from the wisdom of the “crowd”. There are countless success stories in which citizens participate, contributing with their knowledge, cognitive capacity, creativity, opinion, and skills. However, for many scientists, the lack of familiarity with the particular nature of citizen participation, which is usually anonymous and volatile, turns into a barrier for its adoption. This chapter presents a problem-based typology for citizen-science projects that aims to help scientists to choose the best strategy for engaging and counting on citizen participation based on the scientific problem at hand; and some examples are included. Moreover, the chapter discusses the main challenges for researchers who intend to start involving the citizens in order to solve their specific scientific needs.