Metal-halide perovskites have been explored for a wide range of applications, most notably photovoltaics, sensing and lighting. The properties of this class of semiconductors with great chemical and structural diversity can be tailored for specific applications by tuning their photophysics through, e.g., chemical substitution and dimensional reduction. Here, I will show how we use first-principles techniques such as density functional theory (DFT) and Green’s function-based many-body perturbation theory to gain an atomistic understanding of their electronic and excited state structure and provide design rules for bespoke material properties. I will focus on two classes of heterogeneous perovskites: 1. Halide double perovskites, which feature highly tunable band structures dominated by contributions from two different metals sites. We unravel the atomistic origin of the strongly material-dependent optoelectronic properties of these semiconductors, in particular their excitons with binding energies varying by several orders of magnitude, which are ill-described by canonical models – a consequence of their chemical heterogeneity. 2. Quasi-twodimensional (2D) halide perovskites, incorporate large organic molecules such as butylammonium or phenylethylammonium and can be thought of as layered derivatives of their 3D congeners. Excitons in these materials are governed by confinement effects. However, their thermal and electronic properties are a result of static and dynamic distortions arising from an interplay between organic and inorganic layers, which we show by using DFT and molecular dynamics simulations with machine-learned force-fields, supported by photoemission, Raman spectroscopy and thermal measurements.