I.- Testing Theories of Scientific Change.- II: Case Studies.- 1. 17Th-Century Mechanics.- Galileo’s Copernicanism and the Acceptability of Guiding Assumptions.- Newton’s Rejection of the Mechanical AEther: Empirical Difficulties and Guiding Assumptions.- The Vortex Theory of Motion, 1687–1713: Empirical Difficulties and Guiding Assumptions.- 2. Chemistry from the 18th to the 20th Centuries.- The Chemical Revolution: Shifts in Guiding Assumptions.- Molecular Geometry in 19th-century France: Shifts in Guiding Assumptions.- Kekulé’s Benzene Theory and the Appraisal of Scientific Theories.- Fermentation Theory: Empirical Difficulties and Guiding Assumptions.- The Polywater Episode and the Appraisal of Theories.- 3. 19Th-Century Physics.- Ampère’s Electrodynamics and the Acceptability of Guiding Assumptions.- Brownian Motion and the Appraisal of Theories.- The Michelson — Morley Experiment and the Appraisal of Theories.- 4. Recent Geological Theory.- Plate Tectonics and Inter-Theory Relations.- The Theory of an Expanding Earth and the Acceptability of Guiding Assumptions.- 5. 20th-Century Physics.- Planck’s Quantum Crisis and Shifts in Guiding Assumptions.- Nuclear Magnetic Resonance and the Acceptability of Guiding Assumptions.- Electroweak Unification and the Appraisal of Theories.- Index of Thesis Citations and.