In 2012, National Research Council published a new science education framework that explains the science practices and its importance in understanding the process of knowledge development. The students were expected to engage in all the practices by grade 12. All science teachers need some kind of support to improve their understandings of these science practices (NRC, 2012). An important key component of engaging teachers in scientific investigations is to have the teacher participate in a research laboratory experience (NRC, 1996). Research Experiences for Teachers programs (RET) serve as a promising form of professional development to achieve this goal. These programs allow teachers to experience scientific inquiry. The context of the study was a Research Experiences for Teachers (RET) program supported by National Science Foundation It was located in an United States university Environmental Engineering program. There were five preservice and ten inservice teachers in this research. In addition, there were six professors and eight graduate students who served as mentors. Each teacher worked with a specific professor and graduate student mentor in their research projects that are related to the management of the nitrogen cycle, provision of clean water, or urban infrastructure improvement. Also, four professors from engineering and science programs were interviewed to find out what each science practice means to them. The research design of this study was mixed methods that combined quantitative and qualitative research approaches into a single study. In this study, two teachers were selected for the case study based on their experiences and improvements. The study utilized different data sources such as surveys, interviews, observations, and documents. Each research question was addressed based on the results of overall analysis of all the teachers, as well as the results of each case. To find out whether there was a statistically significant difference between the pre-, mid-, and post teacher surveys, repeated measures ANOVA was used for each item. In addition, for the items that showed a statistical difference a Tukey test was conducted to find -which surveys -were significantly different from each other. Also, partial eta squared effect size was calculated for each item. Professor and graduate students' surveys were analyzed by a repeated measures ANOVA and Tukey tests. All interviews were transcribed by the researcher. The data from the interviews were coded and analyzed using a qualitative analysis software. In order to analyze the data in the observations, coding of the qualitative data procedure was employed. The teachers' poster presentations were evaluated by using the researcher-created rubric that has the criteria for each expected part of the poster presentation. The findings of this research suggest that teachers have naive understandings of science practices before they participate in an RET program because they do not have opportunities to learn what those practices mean. The findings also suggest that the teachers still have naive understandings after they participate in an RET program. This is a very important contribution to the literature, in that it is difficult for the teachers to teach those practices in their classrooms if they do not have complete and appropriate understandings of what those practices actually mean. The findings also indicated that teachers' participation in the RET program helped them to improve their abilities to engage in science practices but they need more experience, knowledge, and abilities to engage in the specific practices where they had least improvements. The analysis also indicated that the teachers who actively engaged in the science practices, had productive discussions with the graduate student mentors and participated the quick lessons they gave, read the literature for their research, used new techniques and methods, and participated in the research group meetings improved more on the abilities of engaging in science practices compared to the teachers who did not have the opportunity to participate in the practices because of the structure of their projects, had low interest, and received most of the information directly from the graduate students.