In the last few decades, the large-scale evaluation has acquired an importance in the educational field. International studies developed from surveys are increasingly numerous and cover different domains. Portugal has been participating in several studies, allowing to obtain information about the educational system and the socioeconomic context of the families and students. These data are reflected in the definition and implementation of educational policies. In this book, we present an analysis of three international studies in the field of education, in which Portugal participates: Trends in International Mathematics and Science Study (TIMSS), Progress in International Reading Literacy Study (PIRLS) and the International Computer and Information Literacy Study (ICILS), carried out by International Association for the Evaluation of Educational Achievement (IEA). In this book, we present an analysis of the general results of Portugal and we use an interpretative methodology, based on the analysis of reports and databases produced by different organizations, and undertaken by IEA. Despite the different fluctuations, in the different study cycles, Portuguese students have been showing positive results.