The Center for Pre-College Programs at New Jersey Institute of Technology sponsors a series of summer enrichment programs designed to increase academically talented students' interest in the fields of science, technology, engineering and mathematics (STEM). Programs such as these can be instrumental in informing young students about careers in engineering and technology and helping ensure they receive the academic background required to prepare for these careers in college. One of the programs has been designed specifically for young girls in an effort to increase the number of women interested in engineering and other technological careers. Although there is much debate about the relative effectiveness of female-only programs, previous research comparing aspects of our female-only program to equivalent mixed-gender programs has shown that they can be particularly effective in reaching young girls, influencing their perceptions of engineers and attitudes toward engineering as a career. The addition of equivalent male-only programs has prompted additional research comparing changes in students' perceptions of engineers and attitudes toward engineering, as well as increases in learning and content knowledge. The three different gender grouped programs (female-only, male-only and mixed-gender) were identical in content. The single gender programs within the same grade were taught by the same teacher. Although the mixed gender programs were taught by a different teacher the curriculum was exactly the same. In addition to objective measures of effectiveness, teachers were interviewed to collect qualitative data about the climate in the classroom and students' interactions within the three different gender groupings. Results show differential effects among the programs, not only in terms of perceptions, attitudes and learning, but also in the classroom climate and students' interactions in the classroom.