Epistemological beliefs, or beliefs about the nature of knowledge and knowing, are currently a target of increased research interest. The present study examined two research questions: (1) how do epistemological beliefs change over time? and (2) what role do gender, ethnicity, SES, and achievement play in their development? The study was correlational with an ethnically diverse sample of 187 fifth grade students (46% Latino, 27% Anglo, and 27% African American, and 67% low SES). Self-report questionnaires that tapped four dimensions of beliefs (source, certainty, development, and justification) were given to students at two time points during the course of a nine-week science unit. Results showed that students became more sophisticated in their beliefs about source and certainty of knowledge over time, but that there were no reliable changes in development and justification. There also were no main or moderating effects of gender or ethnicity, but there were main effects of SES and achievement. Low SES and low achieving children had less sophisticated beliefs in comparison to average SES and high achieving children. There were no significant interactions between gender, ethnicity, SES, and achievement for any of the four belief measures. Results are discussed in terms of personal and contextual factors and their role in the facilitation of epistemological belief development.