The importance of engaging students with complex societal challenges has led to the adoption of various interdisciplinary teaching and learning practices in both K-12 and higher education. However, interdisciplinary learning is one of the most complex domains of contemporary educational practice, and, despite its significance, remains significantly undertheorized and under-researched. This Special Issue highlights empirical research efforts toward understanding interdisciplinary learning in its complexity. It simultaneously aims to (1) advance ecological perspectives that encompass concepts and methodologies for studying complex heterogeneous learning practices and (2) apply these perspectives to the research of interdisciplinary learning - of how people learn across and beyond disciplines. This introduction provides a historical context for interdisciplinary learning, introduces an ecological stance toward researching learning across and beyond disciplines, and reviews critical theoretical and methodological challenges within interdisciplinary learning, arguing that the field of the learning sciences is well-positioned to address these challenges. It discusses how the contributions presented in this special issue shed light on theoretical, methodological, empirical, and design aspects of interdisciplinary learning and offer a basis for further design work and research.