STEM integration has attracted considerable attention in educational research, yet there has been a dearth of research exploring how STEM integration is perceived and implemented in regular school settings. The purposes of the study were to explore Taiwanese junior secondary teachers' conceptions of STEM integration with regard to their practices, investigate the ways STEM integration is implemented in the current school curricula, and analyze contextual supports and barriers to the implementations. This study adopted a qualitative research design. Semi-structured interviews were conducted with 19 in-service teachers who delivered integrated STEM in their classrooms. The teachers' conceptions and contextual factors were analyzed using the constant comparison method, whereas the implementations were examined by the two-dimensional spectral framework proposed in the study: "disciplinarity" and "pedagogy." Three categories of teacher conceptions were identified that highlighted how STEM can bring different learning experiences to students. The implementations varied in the degree of disciplinary integration and the extent to which teacher guidance was provided. While school conditions and wider educational contexts posed challenges to STEM integration and implementation, school professional support plays a key role in facilitating integration and teaching across disciplines. This study provides insights into how teachers reconcile their conceptions with school conditions and educational contexts to implement STEM integration in regular classrooms. The two-dimensional framework remarks the values of diversity, namely, to appreciate the learning possibilities opened by varied integration approaches, promote diverse learning outcomes, and allow adaptations based on school contexts.