Investigations of the oxygen carriers range from the characterization of natural populations to measurements of tenths of nanometer distances between atoms. The scope is so great that few biologists and biochemists can fully comprehend the primary literature in its entirety. In addition, the findings of the past two or three decades have advanced the field so rapidly that a truly current account is not readily accessible to a general audience. In recognition of the problem a symposium was held and its proceedings published in the American Zoologist in 1980. Although it included several research reports, most of the contributions were intended to summarize then state-of-the-art information on molecular structure and respiratory function at a level that could be understood by biologists and biochemists who are not experts on our subject. Judging from the reprint requests with which the authors were inundated, the assessment of need had been accurate. I believe that the need for an update, which is wholly focused on communication to the general audience, is even greater in 1992. I therefore asked the authors of this volume to address individuals who might otherwise turn in vain to an advanced textbook of physiology or biochemistry. I have, of course, requested a more comprehensive coverage than would be possible in a general text, but one that is not more parochial. Just as textbooks differ vastly in the level at which their subject matter is presented, so the level of non-expertise was conceived differently by the contributors to this volume.