Hans Ågren develops theoretical models, in which every electron, atom and molecule is accounted for, and he interprets the results of these models in terms of chemical structure, properties and dynamics. He establishes methods that combine quantum and classical physics, where the accuracy of the former is joined with the applicability of the latter. He explores the interaction of light and matter at different scales in time and space and uses the results to predict and interpret molecular and spectroscopic properties and to design molecular probes that operate at different wavelengths, for instance spin-labels that build on magnetic resonance effects in the radio-frequency region, vibrational fingerprints given by the Raman effect, optical absorption and fluorescence relying on the two-photon principle and PET tracers that operate at the wavelengths of gamma-rays. These quantum-classical approaches have applications in a wide variety of applied research areas, in chemistry, biotechnology, biomedicine and material science.