This chapter presents the design and implementation of the International Computer and Information Literacy Study (ICILS) 2023 computational thinking (CT) test instrument, highlighting its focus on assessing students’ ability to conceptualize problems and operationalize solutions. Computational thinking is defined as the capacity to recognize and formulate real-world problems for computational solutions and to develop algorithmic approaches that can be executed by a computer. The ICILS CT framework organizes the CT construct into two strands: conceptualizing problems and operationalizing solutions. The CT test instrument includes four 25-minute modules designed to evaluate competencies such as problem analysis, algorithm development, and solution evaluation. These tasks reflect authentic scenarios, allowing students to demonstrate skills like block-based coding, simulation use, and algorithm debugging. The ICILS CT achievement scale categorizes student performance into four described proficiency levels, ranging from fundamental sequencing through to systems thinking. Methodological details explain how the achievement scale was constructed and equated across cycles, ensuring consistency and comparability. The chapter concludes with illustrative examples from the released test materials, showcasing specific tasks and their alignment with the CT framework, offering insights into how educators can leverage these tools to enhance teaching and support student progress in computational thinking.