Educational choices made by 15 year olds represent a high stake moment in the Swiss highly differentiated tracked school system. This thesis thus proposes an empirical analysis of upper secondary school transitions under four different angles using exhaustive registry data for a single Swiss Canton, Geneva. Chapter 3 assesses how transition opportunities are affected (and constrained) by social capital endowments and migration background. Chapter 4 looks at the impact of the age at first tracking on the smoothness of subsequent upper secondary school transitions. Chapter 5 focuses on transitions to apprenticeship training and examines apprentices' pathways. Chapter 6 deals with transitions across modes of training within vocational education and training, examining the impact of changing modes of training on the qualification probability on the one hand, and on the number of years necessary to qualify on the other.