Historians and philosophers of science have recognized the importance of controversies in the progress of science. The objective of this study was to facilitate in-service chemistry teachers' understanding of conceptual change based on alternative philosophical interpretations (controversies). Selected controversies formed part of the chemistry curriculum both at secondary and university freshman level. The study is based on 17 in-service teachers who had registered for all week course on "Investigation in the Teaching of Chemistry" as part of their Master's degree program. The course is based on 17 readings drawing on a history and philosophy of science perspective with special reference to controversial episodes. Course activities included written reports, class room discussions based on participants' presentations, and written exams. A major finding of this study is that most teachers went through an experience that involved: inconsistencies, conflicts, contradictions, and finally some degree of conceptual change. A few of the participants resisted any change, but still raised important issues with respect to conceptual change. Some of the educational implications are: a) Similar to a scientist, a student can live with two rival theories simultaneously and as the student enriches his cognitive repertoire the conflict can perhaps be resolved; b) Resolution of a conflict may not follow a logical pattern of reasoning but rather a slow process (based on motivational, intuitive, and affective factors) in which the hardcore of beliefs slowly crumbles; c) In science there is no absolute truth, nor a scientific method and consequently there cannot be rules, methods, algorithms, or pre-determined steps for introducing conceptual change; d) Teachers' epistemological outlook is crucial in order to facilitate conceptual change.