The considerable attention paid to metal halide perovskites (MHP) in recent years led to several scientific and technological advances, among which the creation of a new photovoltaic (PV) technology that competes with the most widespread ones[1]. With the ever-increasing diversity of applications explored, the need for fundamental understanding persists and may be gained either by smart and simple experimental protocols or models, or else via complex set-ups and computationally expensive ab-initio calculations. At present, MHP materials used in devices are frequently far from their ideal bulk structure, which hinders the use of state-of-the-art first principles simulations including many-body effects and calls for the design of dedicated models. For PV and LEDs, they are mixed with each other in complex alloys and heterostructures, including 2D/3D compositions, combined with additives or protecting layers to improve their stability and/or defectiveness as well as assembled with carrier selective layers. On the other hand, colloidal MHP nanocrystals are promising for devices exploiting both weak and strong light-matter interactions, such as lasers, single photon emitters if not all-optical logic gates. After a general introduction on MHPs, the talk will bring up some of the hot topics in the field and will summarize recent results obtained by the group in Rennes in collaboration with our partners from France and abroad[2-6]. References :  [1] https://www.nrel.gov/pv/cell-efficiency.html and https://www.nrel.gov/pv/module-efficiency.html. [2] W. Li et al., Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells, Nature Nanotechnology, 17, 45 (2022) [3] B. Traore et al., A theoretical framework for microscopic surface and interface dipoles, work functions and valence band alignments in 2D and 3D halide perovskite heterostructures, ACS Energy Lett. 7, 349 (2022) [4] B. Traore et al., Band gap and energy level alignment of 2D and 3D halide perovskites using DFT-1/2, Phys. Rev. Materials, 6, 014604 (2022) [5] S. Sidhik et al., Deterministic fabrication of 3D|2D 1 perovskite bilayer 2 stacks for durable and efficient solar cells, accepted. [6] Q. Akkerman, et al., Taming the nucleation and growth kinetics of lead halide perovskite quantum dots, https://doi.org/10.21203/rs.3.rs-1236393/v1