This book offers an inside look into the notoriously tumultuous, professional relationship of two great minds: Karl Popper and Paul Feyerabend. It collects their complete surviving correspondence (1948-1967) and contains previously unpublished papers by both. An introduction situates the correspondence in its historical context by recounting how they first came to meet and an extensive editorial apparatus provides a wealth of background information along with systematic mini-biographies of persons named.