Internalization theory has provided a resilient analytical framework that explicitly or implicitly underlines much of International Business scholarship. Internalization theory is not a monolithic body of knowledge; instead, it has devolved into several 'streams', each of which focuses on the interests of particular epistemic communities, while also acting as a more generic organizing framework for those more broadly interested in its application to real-world challenges. Following a review of the various streams, we trace the frontiers of current research of the broader internalization framework and identify emerging themes raised by the papers in the special issue. These include transaction cost considerations in the bundling and recombination of assets across diverse contexts, the growing relevance of quasi-internalization, the theoretical challenges of (bounded) rationality for internalization theory, and the increasing disconnect between ownership, control and responsibility. These developments point to new research frontiers for scholars looking to apply or advance internalization theory.