Excimers are a peculiar class of molecular aggregates that are characterized by a completely repulsive or only weakly binding ground state, while association becomes largely favorable in the excited states. Their formation thus requires the system to be photoexcited. However, the corresponding absorption spectrum is indistinguishable from that of the isolated constituents, as the concentration of ground state dimers, if any, is negligible. Although the term excimer is sometimes used to encompass all of these excited state aggregates, it should be more appropriately reserved for dimers composed of two identical monomers. In contrast, those formed by different chemical species are referred to as exciplexes. Initial studies on excimers date back to the 1960s, when a large amount of experimental evidence and data (including binding energies, lifetimes, and kinetic constants) were extracted from both steadystate and time-resolved fluorescence measurements. Indeed, the hallmark of excimer-exciplex formation is a broad, structureless fluorescence band that appears at higher wavelengths compared to the typical emission of the individual constituents: this red-shift, together with the absence of a vibronic substructure, is related to the larger stabilization experienced by the nuclear conformation at the bottom of the S1 state, which does not correspond to a well-defined minimum in the ground state potential energy surface (PES).