Building epitaxial interfaces between two materials that match each other with atomic precision is key to control the flow of electrons in many technological devices spanning electronics, optics and catalysis. Today, these interfaces are realized also with colloidal nanocrystals, for example in the strongly light emitting core/shell quantum dots used in TV displays. For nanocrystals, the synthesis of epitaxial interfaces based on traditional semiconductors (metal chalcogenides/pnictides, etc.) is well consolidated, while it has been much more challenging with metal halides (including the popular halide perovskites), for two reasons: (i) the attempt of coupling materials that are structurally very different from each other; (ii) the high reactivity of metal halide nanocrystals that defies conventional approaches to make heterostructures. This is regretful, considering that many applications (in lighting, energy conversion, catalysis, etc.) would greatly benefit from the ability to grow heterostructures, also considering the variety of materials belonging to the metal halide family. In NEHA, I will turn the intrinsic reactivity of metal halide nanocrystals into an opportunity to re-design synthetic strategies of nanoscale epitaxial nano-heterostructures in which at least one component is a metal halide. I will leverage on our recent discovery that these heterostructures can form when there is a continuity of ionic sublattices, ensuring that the local coordination of ions at the interface is similar in both components. My aims are to: i) identify materials that can be coupled to form epitaxial heterostructures; ii) uncover the synthesis conditions to make these nano-heterostructures; iii) study their properties, also with advanced techniques and modelling, and transformative behaviour; iv) exploit them in proof-of-concept applications that will benefit from the presence of metal halide interfaces. These will include photocatalysis, photoharvesting and photonic devices.