This book offers an innovative perspective on trilingual education (English, Spanish, and Catalan) in primary schools through the CLIL (Content and Language Integrated Learning) approach. Drawing on a large-scale study in the Valencian Community, it explores how learners develop academic writing across three languages and how factors such as attitudes, identity, language at home, and extramural exposure shape their written performance. Moving beyond the debate on "CLIL intensity," the book highlights the voices of students, families, and teachers, while providing practical tools for schools: a whole-school self-assessment rubric for CLIL, a Science-specific rubric based on Cognitive Discourse Functions, and a teacher reflection checklist. By combining theory, empirical evidence, and classroom applications, the volume offers valuable guidance for researchers, teachers, school leaders, and policymakers seeking to promote multilingual education that is inclusive, research-informed, and sustainable.