The objective of this thesis is to examine the role of conceptual problems in scientific change, especially in the processes of theory appraisal. In the thesis I begin with a review of Buchdahl's, Toulmin's, and Laudan's works on conceptual problems. In the review I show that, although all these writers emphasize the importance of conceptual problems as a criterion of theory appraisal, their works on conceptual problems are not complete. The basis of this thesis is a case study of the nineteenth-century optical revolution. Traditionally, the victory of the wave theory in the revolution was supposed to be due to its empirical successes. However, historical research, presented here, does not support this opinion. I present a different view of the optical revolution, comparing the conceptual problems of wave and particle optics, and identifying the appraisal criteria that historical figures actually employed. I argue that the inferior status of the particle theory in dealing with conceptual problems was the primary cause of the optical revolution. Based on a generalization of a variety of historical cases of conceptual problems, I offer a new account of conceptual problems. First, conceptual problems are the characteristics of conceptual structures rather than theories. Second, the sources of conceptual problems are the processes of concept application, especially in identifying the existence of a concept's referent, in specifying the properties of its referent, and in explicating the procedure of its application. Third, the primary symptom of conceptual problems is that a conceptual structure becomes meaningless. In conclusion, I present a comprehensive set of categories for classifying conceptual problems. / Master of Science