In 1884 Sir William Thomson (later Lord Kelvin) delivered a significant series of lectures on physics at the Johns Hopkins University in Baltimore. The lectures remain important because, through their explicit presentation of the theories and metaphysical assumptions of the Newtonian mechanistic tradition, they illuminate the roots of the revolution in physics that began around 1900. This book presents the twenty lectures in their original form for the first time. (A greatly revised version of the lectures appeared in 1904.) In addition, it contains ten original essays in which well-known historians and philosophers of science discuss the physical issues raised in the Baltimore Lectures and developments in theoretical physics since they were delivered. Several of the accompanying essays deal with differences between Kelvin's views of molecular dynamics and the wave theory of light and the ultimately more successful electromagnetic concepts of James Clerk Maxwell. Others consider G. F. FitzGerald's approach to the question of mechanical models and Ernest Rutherford's attitudes toward theoretical matters. The philosophical context of the Baltimore Lectures is taken up, along with the subsequent development of theoretical physics. Several essays reflect upon issues important in the era of relativity and quantum theory - among them the quantum-measurement problem, space-time and action at a distance, parts and wholes, locality and nonlocality, and the transition from natural philosophy to the metaphilosophy of science. Following an introduction by Robert W Kargon, the essayists who address theoretical physics in Kelvin's time and after are P. M. Harmon, Bruce J. Hunt, M. Norton Wise, Crosbie Smith, Howard Stein, Lawrence Badash, Abner Shimony, Paul Teller, John Earman, Arthur Fine, and Thomas Nickles. The editors are affiliated with the Center for the History and Philosophy of Science at the Johns Hopkins University. This volume is the second in a series published by The MIT Press for the Center. The first volume, Observation, Experiment, and Hypothesis in Modern Physical Science, edited by Peter Achinstein and Owen Hannaway, was published in 1985. A Bradford Book.