This study explores in-service elementary and secondary science teachers' conceptions of the Nature of Scientific Inquiry and the influence of participation in two different Research Experience for Teacher (RET) programs had on these conceptions. Participant teachers attended one of two six week RET programs in which they worked with scientists to engage in scientific inquiry. Before and after the RETs, teachers completed the Views of Scientific Inquiry (VOSI) questionnaire. Teachers' answers were analyzed to determine the degree of sophistication of their understanding of five facets of scientific inquiry. Both elementary and secondary teacher participants showed improvement in their understanding of nature of scientific inquiry as a result of program participation, and both programs were successful in supporting the development of inquiry conceptions, although secondary science teachers started and finished the RET's with a more sophisticated understandings of scientific inquiry. Areas of improvement for elementary teachers included the role of questions in science and the role subjectivity and creativity play in the processes of science, and for secondary teachers growth was seen in the role of questions, the relationship of data and evidence, the distinction of experiments and other means of investigations, and the varied methods of science. Implications of these results are discussed.