The chemistry of light-matter states (polaritonic chemistry) is a relatively new research area in chemistry. Recent experiments have demonstrated that molecular polaritons can have a profound impact on the outcome of chemical reactions taking place inside cavities. Molecular polaritons are formed when the molecular degrees of freedom couple strongly with the modes of a quantum field. From a theoretical point of view the experiments are highly complex, with many different interactions taking place, and a detailed theoretical understanding of the observations is still uncertain. The mission of QuantumLight is to explore, using advanced theoretical modeling, the phenomena that arise when quantum fields interact with molecules and the possibilities that emerge for chemistry. Detailed theoretical and computational understanding of these phenomena will open completely new ways to control and manipulate molecular systems and study new states of matter. The theoretical foundation is cavity quantum electrodynamics (QED), and it will, when combined with the methodologies of quantum chemistry, enable a predictive computational framework for interpretation and future design of polaritonic chemistry. The QuantumLight project will develop and apply accurate electronic structure methods for molecules interacting with quantum fields, in particular coupled cluster theory. Different types of quantum fields will be studied, focusing on those that appear inside optical cavities and the surface plasmon polariton field that is formed by metallic nanoparticles and nanogaps. Applications of the methodology will include ultrafast dynamics in photochemistry, molecules in chiral cavities, electron-photon dynamics, X-ray spectroscopy in cavities, and polariton-assisted chemical reactions.