This paper presents a review of the principles of molecular replacement with the audience of the CCP4 Study Weekend in mind. A complementary presentation with animated Patterson maps is available online (http://cci.lbl.gov/-rwgk/ccp4sw2001/). The implementation of molecular-replacement methods in the Crystallography and NMR System (CNS) is presented and discussed in some detail. The three principal components are the direct rotation function, Patterson correlation refinement and the fast translation function. CNS is available online and is free of charge for academic users.