This chapter aims to review and investigate the measure of environmental knowledge of the Trends in International Mathematics and Science Study (TIMSS) science assessment. Particular focus is placed on the distinctness of general and subscale scores, including the newly developed environmental knowledge scale. The chapter begins with a summary of the TIMSS science assessment framework and its psychometric scaling procedure. The subsequent section outlines the process for constructing the TIMSS 2019 environmental knowledge scale, commencing with the definition of the construct, the identification of environmental items, accompanied by illustrative examples, and the scaling of environmental knowledge data. Following this, a summary of the average environmental knowledge achievement and scale score distributions across countries is presented. Finally, the dimensionality of the TIMSS 2019 science assessment was evaluated. It was specifically tested whether the data can be explained by a unidimensional item response theory model or a bifactor model. Examining information criteria, subscale reliabilities, and explained common variances evidence is found supporting both the existence of a strong general science factor and nested factors for environmental knowledge and other science subdomains. Implications of the findings for TIMSS subscale reporting are discussed.