Although there are initiatives to attract individuals with STEM degrees and experiences into teaching, little is known about the characteristics and development of qualified professional scientists with previous uncertified teaching experiences becoming teachers. One approach to prepare second career certified teachers (SCCTs) for teaching is through teaching fellowships. This qualitative case study investigated the experiences and development of three SCCTs who were accepted into a one-year, intensive teacher fellowship preparation program. The study focused on the SCCTs development and transformations over the fellowship year. The data collected consisted of questionnaires, interviews, and teaching observations. Cross-case synthesis revealed similarities and differences in the coherences and tensions experienced by the SCCTs throughout the clinical preparation year. In addition, the SCCTs experienced shifting identities from scientists, to students, to teachers, to professionals. This study suggests that science teacher educators need to recognize the diverse prior experiences of SCCTs and assist them in balancing and negotiating tensions that might exist between previous and current teaching experiences.