This study explores college students' representations about the nature of theories during their enrollment in a large astronomy course with instruction designed to address a number of nature of science issues. We focus our investigation on how nine students represent their understanding of theory, how they distinguish between scientific theories and non-scientific theories, and how they reason about specific theories. Students' notions of theory were classified under four main categories: (1) hypothesis, (2) idea with evidence, (3) explanation, and (4) explanation based on evidence. Students' condition for deciding whether a given idea is a scientific theory or not were classified under six criteria: content domain, convention, evidence, mathematical content, methodology, and tentativeness. Students expressed slight levels of variation between their reasoning about scientific theories in general and specific theories they learned in the course. Despite increased sophistication in some students' representations, this study affirms the complex dimensions involved in teaching and assessing student understanding about theories. The implications of this study underscore the need to explicitly address the nature of proof in science and issues of tentativeness and certainty students associate with scientific theories, and provide students with more opportunities to utilize the language of science.