How many drug targets and modes of action are there? It has been estimated that currently available drugs act on approximately 600–700 targets. Optimistic projec­tions suggest that this number could be increased by afactor of10. But this number is still small compared to the diversity of proteins that play arole in our organ­ism. Our genome has been sequenced. We know that the number of our coding genes (about 21,500) is much smaller than originally thought (Sect.12.3). However, the number of relevant proteins encoded by these genes is much larger because, among other reasons, versatile posttranslational modications and alternative splicing cause the genetic information to be diversied into mul­tiple protein variants. So our genome is mapped, but do we know the function of each gene? How can predic­tions about proteins and their functions and possible roles in pathophysiology be extracted from this ood of sequence information? Many of the proteins discov­ered in the genome can be assigned to protein families based on sequence comparisons. Nevertheless, asig­nicant portion of our genetic information still awaits annotation. The rst step has been taken, but what are the spatial structures of these proteins for which only sequences are known? Which ligands are recognized by these proteins and what is their biochemical role in our organism? The biochemical function, that is whether aprotein is aprotease, an ion channel, or atransporter, still does not provide information about the systemic role of the protein in the functional processes of acell or an entire organism. The spatial structure of apro­tein is responsible for this function. For this reason, the spatial structures of the proteins in our genome are intensively studied or their geometries are predicted by computer methods. The goal is to map the structural space of all proteins as well as possible. Then it might be possible to nd aspatially resolved and sufciently homologous reference structure for each discovered se­quence. Today, the structures of all members of afew gene families have already been determined. It is only amatter of time before we have the spatial structure of all relevant proteins. The road may be long and ardu ous, but it is clearly marked out. Will this revolutionize the market for potential drugs and enable completely new therapeutic approaches? The chemical space of all conceivable compounds and the biological space of all possible pathology-relevant proteins were discussed in Sects.11.4 and 12.4. Drug design attempts to merge these two spaces. In the cross-section of both spaces, there are molecules to be found as candidates for poten­tial drugs. This chapter attempts to provide an overview of the range of mechanisms of action and some general principles in the families of similar proteins and biomol­ecules. Details of the individual families can be found in Chaps.23–32.