Supportive relationships are key to a healthy, long, and fulfilled life (Umberson & Karas Montez, 2010). While much is known about the general structure of our personal relationships (i.e., ego networks; Sutcliffe et al., 2012) and self-bounded communities (i.e., complete or cohort networks; Zhou et al., 2005), we still know relatively little about how these social networks adapt to critical life events, such as the transition of entering university (Wrzus et al., 2013). This thesis investigates the effects of this first major transition on young adults' personal relationships, social integration, and mental health. In Chapter 1, I introduce the theoretical and empirical background underlying my research and present my unique prospective, longitudinal social network design in which students of two consecutive cohorts at a College of Oxford University participated (five assessments, across 2.5 years in Study 1, N = 90; and seven, across 1.5 years in Study 2, N = 81). In Chapter 2, I examine how these students manage their personal relationships and find that ego networks are overall remarkably robust and grow uniformly within just the first two months at university. In Chapter 3, I investigate how students socially integrate into their new social environment and find that cohort networks driven by homophily factors form rapidly and stabilise just as quickly. In Chapter 4, I study the mental health trajectories of these young adults and find uncommonly high levels of anxiety and depression that temporarily or gradually worsen during the transition. Finally, in Chapter 5, I discuss the main findings and outline their theoretical, methodological, and practical implications. Overall, this thesis shows that the impact of the transition to university on students' social relationships and mental health occurs rapidly. Future research might focus on the first few weeks of this transition in more detail and differentiate students' individual experiences further.