Chemical space is estimated to contain ca 10^60 distinct small molecules; however in practice only tens of thousands (in more complex assays) to hundreds of millions (in DNA-encoded libraries) of compounds can be screened, which is many times smaller. Therefore, the selection of 'relevant' chemical space for choosing which compounds to test is crucial, comprising the topics of virtual screening and library design. In this contribution we will discuss both methods and practical applications of both topics.