The kinetics and thermodynamics of intermolecular excimer formation is disucussed. The temperature dependence of the excimer lifetime, radiative rate constants of excimer (k’_f) and monomer (k_f) and also the solvent density affect the determination of the excimer formation enthalpy ΔH and activation energy E_a, when they are determined from the excimer-to-monomer fluorescence intensity ratio Φ’/Φ. The high - and low-temperature limits (HTL and LTL) of excimer formation are treated and it is shown that there are two ways to stop excimer formation: predominancy of the thermal excimer dissociation (HTL) or suppression of the diffusion controlled excimer formation (LTL). The relative ease to observe excimer emission in the case of pyrene as compared to other aromatic hydrocarbons is related to the ratio k’_f / k_f, which is considerably larger for pyrene than for benzene, naphthalene or anthracene. Time-resolved excimer formation is discussed in detail, based on a kinetic scheme consisting of two excited state species, the monomer ¹M* and the excimer ¹D*. It is argued that transient effects, i.e., time-dependent rate coefficients of excimer formation, do not have to be taken into account. The thermodynamics of intramolecular triplet excimer formation is treated with 1,3-di(9-phenanthryl)- propane as an example. The values for ΔH and E_a are determined from T-T absorption measurements.