The paper presents a theoretical and computational approach to calculate excitonic polarons and self-trapped excitons from first principles, combining the many-body Bethe-Salpeter approach with density-functional perturbation theory. This method does not require explicit supercell calculations, making it computationally efficient. The authors demonstrate their method using Cs2ZrBr6, a vacancy-ordered double perovskite, which exhibits signatures of exciton self-trapping. The study reveals highly localized excitonic polarons, with charge densities that are more diffuse compared to electron and hole polarons. The formation energy of the excitonic polaron is significantly lower than that of the electron and hole polarons, attributed to the partial cancellation of electron-phonon and hole-phonon couplings. The method allows for the investigation of small and large excitonic polarons and self-trapped excitons, providing insights into the physics of exciton-phonon couplings and their applications in solar energy harvesting, photocatalysis, and light-driven quantum matter.The paper presents a theoretical and computational approach to calculate excitonic polarons and self-trapped excitons from first principles, combining the many-body Bethe-Salpeter approach with density-functional perturbation theory. This method does not require explicit supercell calculations, making it computationally efficient. The authors demonstrate their method using Cs2ZrBr6, a vacancy-ordered double perovskite, which exhibits signatures of exciton self-trapping. The study reveals highly localized excitonic polarons, with charge densities that are more diffuse compared to electron and hole polarons. The formation energy of the excitonic polaron is significantly lower than that of the electron and hole polarons, attributed to the partial cancellation of electron-phonon and hole-phonon couplings. The method allows for the investigation of small and large excitonic polarons and self-trapped excitons, providing insights into the physics of exciton-phonon couplings and their applications in solar energy harvesting, photocatalysis, and light-driven quantum matter.