IEA's International Computer and Information Literacy Study (ICILS) 2018 is designed to assess how well students are prepared for study, work, and life in a digital world. The study measures international differences in students' computer and information literacy (CIL): their ability to use computers to investigate, create, participate, and communicate at home, at school, in the workplace, and in the community. Participating countries also have an option for their students to complete an assessment of computational thinking (CT). The ICILS 2018 user guide describes the content and format of the data in the ICILS 2018 international database. It introduces the use of weighting and variance estimation variables for analyzing the ICILS 2018 data, and provides a comprehensive overview of how to work with IEA's International Database (IDB) Analyzer software. The ICILS 2018 user guide is accompanied by three appendices: the international versions of all questionnaires; an overview of national adaptations to the national versions of the ICILS 2018 international questionnaires; and a comprehensive catalogue of the derived variables used in the ICILS 2018 international report. [This guide was written with contributions from: Sabine Tieck, Sabine Meinck, Oriana Mora, and Wolfram Schulz.]