Leonard Culhane has made notable contributions to cosmic X-ray astronomy and solar physics through a wide-ranging series of experimental investigations and the critical analysis of X-ray data. He has also made important contributions to observational techniques. In many highly successful rocket and satellite flights, he carried out significant studies of the solar corona and flares, of binary systems and supernova remnants, and of extragalactic objects. He obtained the first high time resolution observations of solar flare iron lines showing by a penetrating theoretical analysis the importance of conductive cooling in flare–plasma ratio dependence on electron density in the corona. He produced the first X-ray map of the supernova remnants Cassiopeia A and Puppis A, demonstrated that the radiation from the latter was generated in shock-heated interstellar clouds and found an emission line of oxygen VIII from Puppis A. Leonard’s work has established him as a leader of international repute in his subject.