This chapter introduces the fundamentals of audience intelligence’s important aspects. The goal is to present what are related to audience intelligence, how online audience intelligence could be done, and some representative methods. In this chapter, the author will first address the fundamentals of the audience intelligence, including the brief introduction of the online ad eco-system, the relationship between audience intelligence and existing online ad types, performance measures and the challenges in this field. Next, some classical methods of audience intelligence on end-users will be introduces, namely, demographic, geographic, behavioral targeting and online commercial intent (OCI) detection. Then, audience intelligence on advertisers will be presented. Finally, related topics of online advertising, such as the privacy issue, will be addressed.