This book examines the philosophical foundations of the realist view of the progress of science as cumulative. It is a view that has recently been faced with a number of powerful attacks in which successive scientific theories are seen, not as extending their scope and honing their explanations, but as incommensurable. There is, it is held, in principle no way of establishing that they are about the same things. From the voluminous literature on the topic, Dr Smith has selected relevantly and incisively and his exposition of the contending arguments is vigourous and clear, without undue technicality. As an explication and defence of realism it will interest all those concerned with this basic question in the philosophy of science and the philosophy of language.