This study examined the teachers' development as scientists for participants in three National Science Foundation Research Experiences for Teachers. Participants included secondary science and math teachers with varying levels of education and experience who were immersed in research environments related to engineering and science topics. Teachers' functionality as scientists was assessed in terms of independence, focus, relationships with mentors, structure, and ability to create new concepts. Hierarchies developed within these constructs allowed tracking of changes in functionality throughout the 6-week programs. Themes were further identified in teachers' weekly journal entries and exit interviews through inductive coding. Increases in functionality as scientists were observed for all teachers who completed both the program and exit interview (n = 27). Seven of the 27 teachers reached high science functionality; however, three of the teachers did not reach high functionality in any of the constructs during the program. No differences were observed in demographics or teaching experience between those who did and did not reach high functionality levels. Inductive coding revealed themes such as teachers' interactions with mentors and connections made between research and teaching, which allowed for descriptions of experiences for teachers at high and low levels of functionality. Teachers at high functionality levels adjusted to open-ended environments, transitioned from a guided experience to freedom, felt useful in the laboratory, and were self-motivated. In contrast, teachers at low functionality levels did not have a true research project, primarily focused on teaching aspects of the program, and did not display a transition of responsibilities.