This chapter provides a main implication of social class and how the concept of class is defined by different theories of different ages, emphasizing its importance in social division despite that class has been affected by social policy to reduce some inequalities between classes and promote mobility across classes. One key argument of social class is that class division is not constant or fixed, which is explained by giving several examples. In addition, the chapter also suggest that membership of social classes is changing. That is to say, individuals, families or groups may move up and down between classes.