Epistemological understanding is a personal belief about the nature of knowledge (what is knowledge?) and the process of knowing (how does an individual understand the world?). The purpose of the study was to investigate epistemological understanding, especially epistemological thinking about the subjective and objective aspects of knowing, across various judgment domains (aesthetics, value, social world, and physical world) and to see if epistemological thinking varies depending on the grade level and students' fields of study. For this purpose, middle school students in 8th grade, high school students in 10th and 12th grades, and college sophomores participated in the study. The students completed the Epistemological Thinking Assessment (ETA). To summarize the results, the social domain appeared at a higher epistemological level than did the physical domain, indicating that students across grade levels had a more sophisticated epistemological understanding in the social domain than in the physical domain. Significant correlations were found between value judgment and knowledge domains and also between social and physical domains, but the correlations were not very strong, indicating that development does not occur evenly. As predicted, there was a general developmental pattern toward evaluativist thinking in the social domain; but in the physical domain, both absolutist and evaluativist thinking increased as the grade level increased. There were no grade differences in aesthetics and value judgment domains. Also, no significant differences were found in epistemological thinking by students' fields of study. Based on the results, domain-specific aspects of epistemological thinking and the importance of education in its development were discussed.