A content analysis of interviews with an ethnically diverse group of 202 children aged 6 to 10 describes what children think racism is, and examines associations between age, ethnicity, and children's thinking about racism. Children's narratives capture many dimensions of racism, including stereotypes, prejudice, discrimination, and ethnic conflict. With age, children's ideas about racism become more elaborated and differentiated. At every age, compared to their peers, African American children have more elaborated and differentiated ideas about racism and mention those dimensions of racism that overtly reflect power relations more frequently. Qualitative analyses suggest that children's ideas about racism are abstract, increasingly coherent with age, and sometimes incorporate causal language. Findings are discussed in terms of origins of individual differences, the extent to which children's ideas about racism might be considered a lay theory, and the likely consequences of such a theory in daily life.