The quantum yield definitions and formulae have been discussed succinctly in the previous chapters (e.g. Sect. 1.9 ). The importance of determining the correct value of the quantum yield of a reaction, or more precisely of a photoactive species (reactant or product), is essential in photokinetics not only as a means to evidence a possible excitation-wavelength effect (Chap. 4 ) but also to characterise the reactive species, to appropriately model photoreactivity and to predict the evolution of the reaction. We have seen, in the previous chapter, that several equations were proposed by a number of studies for the photochemical quantum yield. This sample is by no means exhaustive of similar formulae found in the related literature. A situation that raises a question about which equation is suitable for the purpose or whether a series of different equations can safely be used to determine the quantum yield of a species. These matters are looked into here with the aim to lift, at least partially, the confusion that might exist around the photochemical quantum yield equation.