Publisher Summary This chapter discusses biological modeling. It remains true that most of the intricacies in response described by Eppley are not incorporated in models, and very simple relations are used for factors such as plant respiration rate where recent evidence suggests complex dependence on other parameters in the plant's metabolism. The second common feature of the models of events in the upper layers of the sea is that the number of state variables is usually small and is usually the same whatever the area studied. These variables are nitrogen, chlorophyll and zooplankton biomass. This gives an impression of generality which may well be spurious, resulting from the inadequacies of the test data. One example of the limitation set by the data is the heterotrophic cycle involving soluble and nonliving particulate organic matter, bacteria and microzooplankton. This neglected system which runs parallel to the main food chain may not merely alter the energy flow but could have important feedback effects for stability.