In this chapter we explain, based on the relevant social science literature, who or what a fan is in the first place and which characteristics and behaviors are typical of fans. We determine why every fan relationship is based on identification and is experienced as unique by the fan. We show that every company can have fans and develop a lean measurement tool that identifies such Fan Customers. Fan Customers, we prove, are highly satisfied and emotionally loyal customers, and we show that our Fan Portfolio makes it possible to segment the universe of customers into five groups ranging from Fans to Opponents. We prove that the Fan Rate, i.e., the percentage of fans among all customers of a company, is the much more valid metric for managing a company than customer satisfaction. At the end, we look at the Fan Rates of different industries in different countries.