Metal halide perovskites are of immense importance due to their excellent optoelectronic properties, holding great promise in the field of advanced and clean energy technologies. One of the fundamental mechanisms governing their physical properties is the interplay of electron-phonon coupling with the strong lattice anharmonicity and their inherent locally disordered structure [1,2]. To understand the consequences of these effects, we have recently introduced a powerful theoretical framework that allows to capture anharmonic electron-phonon coupling in these intrinsically polymorphous compounds [3,4]. In this talk, I will demonstrate our methodology for both inorganic and hybrid halide perovskites and show that (i) local disorder is at the origin of overdamped and strongly coupled anharmonic phonons, (ii) low-energy optical vibrations dominate electron–phonon renormalized band gaps, departing from a simplified picture of a Fröhlich interaction, and (iii) local disorder is the key to explain the monotonic increase of the band gap across phase transitions. Our work provides a new perspective for the interpretation of perovskite materials fundamentals as well as opens the way for efficient simulations of halide perovskites' key properties, like carrier mobilities, excitonic spectra, and polaron physics.