Strong confinement of electromagnetic modes enables frequent energy exchange between light and matter, potentially leading to the formation of hybrid light-matter states known as polaritons. To quantitatively understand how light influences materials, it is essential to treat light and matter on equal footing. In this context, I provide an overview of ab initio quantum electrodynamics (QED) and illustrate its diverse applications, including the mode- and enantiomer-selective control of catalytic reactions. We discuss how strong coupling between optical and vibrational modes can modify chemical reaction rates [1–2], and we demonstrate that charge transfer in plasmonic catalysts can be enhanced by nearly a factor of seven through polaritonic effects [3]. Finally, the theoretical prediction of vibrationally induced resonances in the lasing behavior of organic molecules [4] highlights the potential of ab initio QED to drive progress on both sides of the light-matter interface. [1] C. Schäfer, J. Flick, E. Ronca, P. Narang, and A. Rubio, Nature Communications, (2022) 13:7817. [2] C. Schäfer, J. Fojt, E. Lindgren, and P. Erhart, J. Am. Chem. Soc. 2024, 146, 8, 5402-5413. [3] J. Fojt, P. Erhart, C. Schäfer, Nano Lett. 2024, 24, 38, 11913–11920. [4] K. Müller, K. Luoma, C. Schäfer, arXiv.2405.05093.