Postoperative ICU admission is afforded to patients with high clinical severity, but the benefits and harms of such an endeavour are debateable. The purpose of my thesis was to further understand a) the type of patients being admitted to ICU, b) the role of age in ICU admission, and c) the association (if any) between ICU admission and subsequent mortality and complications. The thesis consists of 4 chapters. Chapter I provides a brief introduction of the topic and the rationale behind the researched questions. Chapter II examines the association between chronological age and ICU admission in postoperative patients. This analysis sheds light on one of the 17 variables included in the score (i.e. age), which drives the clinical severity score in parts of the population. Chapter III uses a propensity score model to match patients that were admitted to ICU and those that were not based on several pretreatment variables, to assess the impact ICU admission has on postoperative mortality and complications. Finally, Chapter IV reflects the conclusions of the thesis and suggests further research agendas. Overall, these three thesis components will illustrate the role of ICU admission in an adult postoperative population as well as its consequences in comparison to those not admitted to an ICU. / Thesis / Master of Science (MSc)