The identification of biologically relevant starting points within a vast chemical space is a particularly relevant task in designing compound collections and selecting compounds from existing libraries for computational and/or experimental screening. Although it is not an easy task, it is possible to utilize the fact that the physicochemical and biological properties of molecules are associated with their molecular structures. This is known as "chemical similarity principle," which states that if two molecules share similar structures, then they will likely have similar bioactivities. Thus, the distribution of the compounds in chemical space guides the search for compounds with a specific set of properties. The choice of descriptors to define chemical space is crucial, however, it is not unique; different from cosmic space, chemical space is not invariant. Therefore, molecular representation is the cornerstone of chemical space (and basically any other computational approach).