Whoever turns to the history of photosynthesis research in the twentieth century is soon confronted with the fact that one of its most exciting periods, the years from 1920 to 1960, was in large part overshadowed by a bitter controversy in which many of the leading scientists in the field were involved. It centered on the question, how efficient the process of photosynthesis was. This book attempts a reconstruction of the course of the controversy, based on previously unknown archival sources, and analyzes the arguments brought forward by the two parties.