The ENS theoretical chemistry group develops an integrated approach of modern theoretical chemistry to describe and understand chemical reactivity and chemical processes in complex environments and on multiple length- and time-scales. Its members combine a broad range of methodologies, including statistical mechanics, quantum chemistry, thermodynamics, analytical theory and numerical simulations, e.g. with molecular dynamics, Monte Carlo and continuum descriptions. Its activities include both theoretical and methodological developments in very close connection with state-of-the-art experiments, often through common projects. These approaches are applied to investigate systems ranging from materials to biomolecules and geological fluids, and address many of the great challenges of chemistry, including energy, environment, green chemistry, and pharmaceutical chemistry.