Strong coupling of light and matter can give rise to a multitude of exciting physical effects through the formation of delocalized hybrid light-matter states. When molecular materials with high transition dipole moments are placed in the confined fields of metallic microcavities or surface plasmons, Rabi splittings approaching 1 eV are observed due to the interaction with the vacuum electromagnetic field. This leads to fundamental changes in the properties of the coupled system even in the dark. While strong coupling has been extensively studied due to the potential it offers in physics such as room temperature Bose-Einstein condensates and thresholdless lasers, the implications for molecular and material science have remained mostly unexplored. After introducing the fundamental concepts, examples of modified properties under strong coupling, such as enhanced transport in organic semiconductors, energy transfer and chemical reactivity, will be given to illustrate the potential of light-matter states.