The gathering of a wealth of precise quantitative observational data on x‐ray binaries and recycled radio pulsars during the last decades has allowed the testing of many theoretical ideas on the physics, formation, and evolution of neutron stars and black holes, which could never have been tested on single compact objects. Also, new observations often have led to new theories and models which again can be tested observationally. In this talk I give a personal view of this development over the past thirty years. ©1994 American Institute of Physics.