This book presents a study on the National Secondary Education Examination (ENEM) and prioritises understanding it as the main selection tool for admission to Brazilian higher education. It includes a recent survey on the researched topic (2010-2015), as well as discussions on the theme of assessment based on classic authors. The main focus is on the historical, structural and functional presentation of the ENEM. Described as an individual and voluntary exam, it is administered annually to students who are completing or have already completed secondary education in previous years. Created in 1998, the exam underwent a significant restructuring in 2009 and today has a total of 18 exams administered. We associate the main characteristics of verification, classification and selection with the ENEM and note that, in addition to its fundamental relevance for admission to higher education, government programmes (PROUNI, FIES, Science without Borders, PRONATEC and Secondary Education Certification) and government systems (SISU and SISUTEC) contribute to the national importance given to the ENEM.