The primary purpose of International Computer and Information Literacy Study (ICILS) 2018 is to assess systematically the capacities of students to use ICT productively for a range of different purposes, in ways that go beyond a basic use of ICT. ICILS 2018 includes authentic computer-based assessments that are administered to students in their eighth year of schooling. These generate data reflecting two dimensions of ICT-related capacities: computer and information literacy (CIL) and computational thinking (CT). ICILS 2018 investigates variations in CIL and CT between and within countries and the relationships between those constructs and student attributes (background characteristics and developed attributes) including their use and experience of computer technologies. ICILS 2018 also investigates how CT is related to CIL. A secondary purpose of ICILS 2018 is to investigate the use of computers and other digital devices by students and teachers, as well as their attitudes toward the use of computer technologies. The ICILS assessment framework articulates the basic structure of the study. It provides a description of the field and the constructs to be measured. It outlines the design and content of the measurement instruments, sets down the rationale for those designs, and describes how measures generated by those instruments relate to the constructs. In addition, it hypothesizes relations between constructs so as to provide the foundation for some of the analyses that follow. Above all, the framework links ICILS to other work in the field so that the contents of this assessment framework combine theory and practice in an explication of "both the ‘what’ and the ‘how’" (Jago 2009, p.1) of ICILS.