Research into athletes' dual careers has been guided by two main approaches. The holistic developmentalapproach has increased our understanding of the demands student-athletes' face and the competences needed tobalance their DC. The holistic ecological approach has shifted the attention to the developmental context andhow DC development environments (DCDE) support or hinder student-athletes' development. So far, DCDEsand their functioning have been described and deemed effective based on general outcome measures (e.g.,sport and academic achievements, dropout rate), but without a clear link to the student-athletes' actual demandsand supportive needs. In this study we explored how a DCDE facilitated student-athletes' transition to, and firstyear adaptation at university level, by combining the holistic ecological and holistic developmental approachesand considering DC balance as a primary concern for DC support. A mixed-methods intrinsic case study wasimplemented to explore the complexity and uniqueness of a specific bounded case, that is, a Scandinavianuniversity-based DCDE and student-athletes' development within it. Inspired by the holistic ecologicalapproach and the DC-environment success factors (DC-ESF) working model we explored the case usingmultiple sources of data. Semi-structured interviews were conducted to explore the transition experiences ofnine student-athletes and the perspectives on the environment and support from four key support providers.This was complemented by documents analysis and a quantitative monitoring of the student-athletes' transitionexperiences (e.g., perceived DC balance, demands, and coping) throughout the educational year. Findings arepresented in a joint display by merging the qualitative and quantitative data in an empirical version of the DCESFmodel. The student-athletes experienced a challenging transition with both athletic and non-athleticdemands. The environment was well-coordinated with support providers centered around a mission of "abalanced and synchronized whole" and targeting empowerment of the whole person. There was convergenceacross sources of information with qualitative and quantitative data showing that the student-athletes improvedin their sport and study, and in their ability to balance their DC while also developing their DC competences.By integrating the holistic ecological and the holistic developmental approaches we were able to situate theenvironment in the student-athletes' development and explicitly show the fit between student-athletes' needs(i.e., perceived demands) and support provided in the DCDE. Based on our finding we believe effectiveenvironments are those that work to optimize their structure and culture to meet student-athletes' needs, helpstudent-athletes develop their DC competencies and maintain their DC balance. © 2021 International Society of Sport Psychology