This article-based thesis draws on research in the fields of science education and international large-scale assessments. It investigates theoretical, methodological, and empirical aspects of inquiry as an instructional approach (means) and the assessment of inquiry as an instructional outcome (ends). The empirical investigations used latent variable modeling in order to analyze data from student and teacher questionnaires, student assessment, and student log files in the Trends in International Mathematics and Science Study (TIMSS) and the Programme for International Student Assessment (PISA) 2015.Article 1 focuses on inquiry as an instructional approach and outcome by exploring the relationship between inquiry-based science teaching and student achievement in science. This article attempts to resolve conflicting findings of inquiry–achievement relationships by demonstrating the existence of curvilinear rather than linear patterns, as previously assumed. Article 2 addresses the research gaps in comparing inquiry as an instructional approach between primary and secondary education. It examines the interplay between teachers’ self-efficacy in teaching science and perceived time constraints in explaining the opportunities for students to engage in cognitively challenging learning activities in Grades 4, 5, 8, and 9. Article 3 presents an investigation on the assessment of inquiry as an instructional outcome. It identifies distinct profiles of students’ performance on simulated inquiry tasks that require the skills to coordinate the effects of multiple variables and to coordinate theory with evidence. While Article 3 takes a micro approach, focusing on specific scientific inquiry skills, Article 4 explores inquiry as an instructional outcome from a macro approach, taking into account a range of formal and informal reasoning skills students need to acquire in order to participate in inquiry practice. This article argues for the importance of assessing formal and informal reasoning and provides a short overview on utilizing the potential of computer-based assessments to assess both types of reasoning.Taken together, the findings presented in this thesis advance the existing knowledge about the important distinction and role of inquiry as a means and an end in science education. This thesis argues that, to understand inquiry in a comprehensive context, it is essential to consider the relationships of the data gathered from various sources: the input, process, and output of inquiry. This study contributes to inform the ongoing science education reform in Norway and to improve the assessment of inquiry as an instructional approach and outcome in international large-scale assessments.