This preface is addressed to the reader who wishes to inquire into the prevailing concepts, hypotheses and theories about development of sensory systems and wants to know how they are exemplified in the following chapters. I believe that science is hypothesis and theory and that the growth and evolution of any branch of science can be measured by the degree to which its theories have been reified. By that standard, one must conc1ude that developmental neuro biologie is in its infancy. The rapid accumulation of observations which has occurred in this branch of science in the past century leads to progress only to the extent that the facts validate or falsify hypotheses. The following chapters show that we have a plethora of facts but a dearth of hypotheses. Another index of the maturity of any branch of science is its level of historical self-awareness. Because the history of any branch of science is essentially the history of ideas and of the rise and fall of theories, the level of historical awareness is related to the extent to which reification of its hypothetical constructs has advanced. It is largely because few theories of development of sensory systems, or indeed, of developmental neurobiology, have progressed far in the process of reification that the his tory of developmental neurobiology remains unwritten. The subject of this volume is hardly mentioned in the many books devoted to the history of related disciplines.