Halide perovskites are of immense importance due to their excellent light-absorbing and emitting properties, holding great promise in the field of advanced and clean energy technologies. One of the main mechanisms governing their unique properties at finite temperatures is the effect of electron-phonon coupling. However, achieving accurate simulations of electron-phonon coupling in these compounds requires understanding the effects of anharmonicity and polymorphism, as well as the intricate nature of their potential energy surface. In this talk, I will show how the anharmonic special displacement method (A-SDM) allows for exploring anharmonic electron-phonon coupling in both layered and bulk halide perovskites. I will first demonstrate the importance of polymorphism in these compounds leading to strongly-coupled vibrational dynamics and strong modifications in the electron-phonon matrix elements . Then, I will discuss that polymorphism holds the answer to elucidating the smooth evolution of the band gap with temperature around phase transitions ; a behavior that has proven challenging to understand thus far. Our work establishes a comprehensive framework that enables precise simulations of halide perovskites' carrier mobilities, excitonic spectra, and polaron physics.