The Trends in International Mathematics and Science Study (TIMSS) is an international assessment that evaluates students’ mathematical and scientific competencies. One component is an assessment of environmental knowledge. This chapter provides an overview of the study’s objectives, the participating entities, sampling design, and the assessment instruments and contextual questionnaires. In TIMSS 2019, 39 countries and seven benchmarking participants were involved at the eighth grade. In each educational system, TIMSS employs a multi-stage sampling approach to select students. Data collection includes achievement assessments in mathematics and science, along with comprehensive background questionnaires for students, teachers, and principals. These instruments are collaboratively developed by participating countries. In summary, TIMSS offers a unique approach to assessing student competencies globally. Its rigorous methodology and careful sampling procedures ensure the quality and comparability of data.