This chapter introduces the fundamental principles of randomized experiments in social networks and highlights the challenges encountered in the research process. Conducting studies using existing online social networks requires establishing collaborative relationships with websites, which can be particularly challenging when dealing with major platforms like Facebook. Building a custom system, on the other hand, is time- and labor-intensive, and the generalizability of experimental results is limited. However, as more websites adopt an open collaborative attitude, the research on social networks based on randomized experiments is expected to progress further.