I.- Testing Theories of Scientific Change.- 1. Preface.- 2. Theories of Scientific Change: A Call for Testing.- 3. A Program for Testing.- 3.1 Formulating Testable Theses.- 3.2 Designing the Test.- 4. Findings.- 4.1. The Prevailing Wisdom about Guiding Assumptions.- 4.2. Theses about Guiding Assumptions.- 4.3. Findings about Guiding Assumptions.- 4.3.1. How They Come to be Accepted.- 4.3.2. The Impact of Empirical Difficulties.- 4.3.3. Innovation and Revolution.- 4.3.4. Summary.- 4.4. The Prevailing Wisdom about Scientific Theories.- 4.5. Theses about Theories.- 4.6. Findings about Scientific Theories.- 4.6.1. Inter-Theory Relations.- 4.6.2. Theory Appraisal.- 4.6.3.Summary.- 5. Conclusion.- Notes.- References.- 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.- Kekul¿ 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.- Amp¿¿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.