The above paragraph reveals some crucial desiderata for a catalyst screening library, namely that it should be unbiased, extensive and exclusively contain valid samples. As we shall see, what this means in practice depends somewhat on the context. Nevertheless, these (partly competing) goals should always be taken into account when designing a screening study. It is the purpose of this perspective to argue that a good way to balance these requirements could be to define the chemical space of interest via formal gram- mars. In the following we briefly give a general introduction into this concept before discussing how it can be useful for catalyst discovery. Formal grammars: Formal grammars were originally developed in the field of theoretical linguistics, where they describe how syntactically valid sentences can be formed from a language's words. An example of this is shown in Fig. .