This monograph is aimed at an analysis of the reasons for theory change in science. The writer develops a new model of theory change according to which the origins of scientific revolutions lie not in a clash of mature theories with facts, but of 'old' mature theories with each other, leading to contradictions that can only be eliminated in a more general theory. The model is illustrated with reference to physics in the early 20th century, the three 'old' theories in this case being Maxwellian electrodynamics, statistical mechanics and thermodynamics. It is suggested that special relativity, both in its creation and its acceptance, can be understood only with reference to developments in quantum theory.