This study (a) assessed the inยฏuence of three history of science (HOS) courses on college students' and preservice science teachers' conceptions of nature of science (NOS), (b) examined whether participants who entered the investigated courses with a conceptual framework consistent with contemporary NOS views achieved more elaborate NOS understandings, and (c) explored the aspects of the participant HOS courses that rendered them more ``effective'' in inยฏuencing students' views. Participants were 166 undergraduate and graduate students and 15 preservice secondary science teachers. An open-ended questionnaire in conjunction with individual interviews, was used to assess participants' pre-and postinstruction NOS views. Almost all participants held inadequate views of several NOS aspects at the outset of the study. Very few and limited changes in participants' views were evident at the conclusion of the courses. Change was evident in the views of relatively more participants, especially preservice science teachers, who entered the HOS courses with frameworks that were somewhat consistent with current NOS views. Moreover, explicitly addressing certain NOS aspects rendered the HOS courses relatively more effective in enhancing participants' NOS views. The results of this study do not lend empirical support to the intuitively appealing assumption held by many science educators that coursework in HOS will necessarily enhance students' and preservice science teachers' NOS views. However, explicitly addressing speciยฎc NOS aspects might enhance the effectiveness of HOS courses in this regard. Moreover, the study suggests that exposing preservice science teachers to explicit NOS instruction in science methods courses prior to their enrollment in HOS courses might increase the likelihood that their NOS views will be changed or enriched as a result of their experiences with HOS.