This chapter defines prejudice and notes some of the many theories that have shaped the study of prejudice development. It summarizes the current empirical findings from research examining racial preferences and prejudice in early and late childhood, and attempts to explain these findings. Two theoretical mechanisms that are helpful for understanding the early high levels of explicit prejudice and implicit intergroup biases include brain maturation and experience with in-group and out-group faces. One interesting way to merge what people know of maturation, whether phrased as brain maturation or cognitive maturation, with social influences such as out-group contact, is to suggest that there are optimal windows of social influence. The concept of an optimal window has been used to explain language acquisition and preference. Exposure to one's mother tongue deletes synapses for foreign languages starting in infancy, so this perceptual narrowing for language may lead to stronger language preferences and prejudices as seen in some research.