Introduction This book contains papers given at a NATO Advanced Research Institute (A.R.I.) held at Caiscais, Portugal, in November, 1981. The subject of the A.R.I. was marine heterotrophy; this is defined as the process by which the carbon autotrophically fixed into organic compounds by photosynthesis is transformed and respired. Obviously all animals and many microbes are heterotrophs but here we will deal only with the microbes. Also, we restricted the A.R.I. primarily to microbial heterotrophy in the water column even though we recognize that a great deal occurs in sediments. Most of the recent advances have, in fact, been made in the water column because it is easier to work in a fluid, apparently uniform medium. The reason for the A.R.I. was the rapid development of this subject over the past few years. Methods and arguments have flourished so it is now time for a review and for a sorting out. We wish to thank the NATO Marine Science Committee for sharing this view, F. Azam, A.-L. Meyer-Reil, L. Pomeroy, C. Lee, and B. Hargrave for organizational help, and H. Lang and S. Semino for valuable editing aid.