This dissertation introduces the construct of "worthwhileness" as an important aspect of students' "practical" epistemologies of science (Sandoval, 2005). Specifically, it examines how students conceptualize what makes a scientific research question worthwhile, through a close analysis of the criteria they use for evaluating scientific research questions. Elementary (n=21) and high school students (n=21) participated in this study. As part of semi-structured interviews, students engaged in three novel tasks designed to elicit the epistemic criteria they use to evaluate scientific research questions in a variety of contexts. Findings indicate that elementary and high school students alike could engage in the practice of evaluating the worth of scientific questions. The criteria they employed included degree of interest, difficulty, and the contribution of questions to knowledge or to solving a problem. The criteria students considered varied by context. Several key differences emerged between the reasoning of the two grade cohorts. High school students tended to place more weight on the contribution of the research question. Also, the criteria reflected in the high school students' judgments of the scientific value of individual questions more closely accorded with the criteria they identified retrospectively as the basis of their judgments. Furthermore, the older cohort more often rationalized the selection and sequence of research questions within a single domain on the basis of epistemic contingency between questions. How students conceptualize what makes a scientific research question worthwhile constitutes a key aspect of students' epistemic reasoning. It is particularly important to understand how students judge the worthwhilness of scientific research questions given the central epistemic role of research questions in scientific inquiry. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]