This text is aimed principally at the beginning graduate or advanced undergraduate student, but was written also to serve as a review and, more ambitiously, as a synthesis of the field. To achieve these purposes, several objectives were imposed on the writing. The first was, since ecol ogists must be the master borrowers of biology, to give the flavor of the eclectic nature of the field by providing coverage of many of the interdis ciplinary topics relevant to marine ecology. The second objective was to portray marine ecology as a discipline in the course of discovery, one in which there are very few settled issues. In many instances it is only possible to discuss diverse views and point out the need for further study. The lack of clear conclusions may be frustrating to the beginning student but nonetheless reflects the current-and necessarily exciting-state of the discipline. The third purpose is to guide the reader further into topics of specialized interest by providing sufficient recent references especially reviews. The fourth objective is to present marine ecology for what it is: a branch of ecology. Many concepts, approaches, and methods of marine ecology are inspired or derived from terrestrial and limnological antecedents. There are, in addition, instructive comparisons to be made among results obtained from marine, freshwater, and terrestrial environ ments, I have therefore incorporated the intellectual antecedents of par ticular concepts and some non-marine comparisons into the text.