The Transition Year programme (TY) is a relatively ‘non-academic’ year embedded midway through the Irish post-primary education system. TY is intended to promote personal development, maturity, social skills, and to prepare students for adult life. Participation rates have been increasing since the 1990s and a majority of students now take part each year, mostly in schools where participation in TY is optional. Previous research, often interviewbased, has found that students, teachers, and parents tend to regard the programme as a positive experience for most students. Participation can enhance students’ sense of maturity, improve relationships with peers and teachers, and develop skills such as time management and the ability to work as part of a team. However, TY is noted as being relatively underresearched, despite being a uniquely Irish innovation. No study has previously sought to measure the extent of any differences in student development that might be attributable to TY participation. This study builds on previous research by gathering quantitative data on a range of psychosocial outcomes and comparing changes over time between students who took part in Transition Year and those who did not. Outcome measures include school engagement, relationships with teachers, self-reliance, work orientation, subjective age, school satisfaction, life satisfaction, and social self-efficacy. These measures are complemented by extensive quantitative and qualitative data relating to students’ direct perceptions and experience of Transition Year. Three waves of longitudinal data were collected, beginning when participants were in Third Year (pre-TY). Participating students were followed up one year later (in TY/Fifth Year) and two years later (in Fifth Year/Sixth Year). 1153 students in 20 schools took part in all three waves, with approximately 5500 students participating in at least one wave. Differences in psychosocial outcomes and experiences of TY are compared and are discussed with reference to previous research. The main conclusions, implications for policy and practice, and avenues for further research are highlighted.