Green hydrogen is feedstock, fuel, energy carrier, and storage at the same time, and one of the important cornerstones to decarbonize industrial and economic sectors on the European continent. The proposed action ‘Towards Green Hydrogen by Layered Metal Halide Perovskite Heterostructures - TOGETHER’ will deliver a highly tunable material platform by integrating lateral heterostructures in two-dimensional layered metal halide perovskites (2DLP) to overcome the high exciton binding energy and ultimately enable long-distance charge separation for photocatalytic generation of green hydrogen. The structures developed in TOGETHER will provide a spatially confined directional flow of electrons to the edge of the semiconducting layer in 2DLPs, where they can be extracted by protons to form hydrogen. The unique flexibility of the materials platform architecture will result in a large degree of freedom to tune each step in the photocatalytic cycle to increase the solar-to-fuel conversion efficiency. The formation of lateral heterostructures in 2DLPs will be achieved through tailored consecutive ion exchange, which is a powerful tool to manipulate the composition while maintaining the crystal structure, size, and shape of the parent object. TOGETHER is a highly interdisciplinary effort that builds on cutting-edge research in material science with chemistry, physics, and engineering.