Chemical space is the number of possible small organic molecules, which has been estimated to be on the order of 10 60 molecules. Many techniques have been developed to search this chemical space for molecules with desirable properties. Rather than screening a user defined library, these methods automatically select a subset of chemical space for screening, usually in an iterative fashion. The size of the subsets typically range between 10,000 and several million, i.e. a tiny fraction of chemical space yet usually produce good candidates. 10 60 -→ 10 3-6 -→ 1