While its advocates trumpet personal epistemology research as an essential contribution to the understanding of student cognition, the field currently wrestles with four problems. There is a lack of consensus regarding construct definition, a disconnect between psychological investigations and personal epistemology's philosophical roots, a failure to integrate work from developmental psychology, and difficulties in measuring personal epistemology. This dissertation combines work from both philosophy and developmental psychology with personal epistemology research to put forth a conceptual model of epistemic and ontologic cognition that addresses these four problems while building on the strengths of past research. Development is described using four ordered positions, and is predicted to be probabilistically related to educational level. Domain-specificity is also tested in terms of ill and well-structured domains. Using both quantitative and qualitative data from a pilot study, an instrument to measure epistemic and ontologic cognition was developed. By assessing the construct validity and reliability of scores from the instrument the underlying conceptual model was tested. This instrument was administered to a sample of 662 students ranging in age from middle-school through graduate school. Results indicated that scores from the instrument had acceptable construct validity and reliability, and that a factor mixture model best represented the data, and provided mixed support for the underlying model. Educational level was probabilistically related to participants' epistemic and ontologic cognition.