A stimulus class can be defined as a group of non-identical stimuli that each occasion a similar response. The primary characteristic is mutual substitutability of class members within a given context and as such each member occasions a similar response. Many stimulus-stimulus relations have been explored and combined into intricate networks of emergent relations. Additional processes such as stimulus generalization, contextual control, and transfer of function combine with derived relational responding to form classes of almost unimaginable size and complexity. The derived relational account has been used to explain more contemporary and socially relevant concepts such as private events, mental disorders, and cultural biases. This chapter will review the literature and provide a partial explanation for prejudice, bias, discrimination, and even racism.