Within the last ten years, the interest of historians and philosophers of science in the epistemological writings of the Polish medical microbiologist Ludwik Fleck (1896-1961), who had up to then been almost completely unknown, has advanced with great strides. His main writings on epistemological questions were published in the mid-1930's, but they remained almost unnoticed. Today, however, one may rightly call Fleck a 'classical' figure both of episte mology and of the historical sociology of science, one whose works are comparable with Popper's Logic of Scientific Discovery or Merton's pioneer ing study of the relations among economics, Puritanism, and natural science, both also originally published in the mid-1930's. The story of this book of 'materials on Ludwik Fleck' is also the story of the reception of Ludwik Fleck. In this volume, some essential materials which have been produced by that reception have been gathered together. We will sketch both the reception and the materials.