This document discusses reference groups, which are groups that individuals use as points of comparison to influence their attitudes, values, and behaviors. It defines membership groups and symbolic groups. Reference groups can be normative, influencing general values and behaviors, or comparative, influencing specific attitudes. The document then describes different types of reference groups classified by membership, interaction level, and other factors. It provides examples of consumer-related reference groups like friendship groups, shopping groups, work groups, virtual communities, brand communities, and celebrities. Finally, it discusses how advertisers may appeal to experts, common people, executives, and employees as reference groups.