Polymorphism in solids refers to the presence of local atomic disorder not discernible in standard diffraction experiments. Although polymorphism is crucial for understanding the electronic structure of halide perovskites, it is frequently overlooked when interpreting their intriguing properties. In this talk, I will present a recently developed ab initio methodology [1,2] for the calculation of transport and optoelectronic properties of perovskites, allowing for the efficient treatment of anharmonic lattice dynamics, electron-phonon coupling, and polymorphism. I will make a connection between theoretical findings and experimental results [3,4,5], beginning with pair distribution functions and expanding to temperature-dependent band gaps, phonons, effective masses, and mobilities. I will also discuss the role of molecular orientation and size in the degree of polymorphism in hybrid compounds, and make contact with the concepts of the lone pair formation and polarons. Our work opens the way for addressing pending questions on perovskites’ technological applications.