This chapter analyzes the evolutionary defensive adjustments of shell-less gastropods, which have been necessary to maintain the arms race. It deals with neither kairomones nor alarm pheromones that will be treated because of their limited contribution against fish predation. In the Opisthobranchia, the general trend toward loss of the shell has been compensated for by various defensive adaptations involving chemical secretions or nematocyst-based defences, generally associated with conspicuous colouration. The opisthobranch defensive mechanisms have been classified into three basic categories: behavioural, morphological, and chemical, which may be employed in a hierarchical fashion in the different stages of fish predation. One way of examining the influence of the opisthobranch colour pattern and chemicals on fish predation and of determining the relative importance of each is by using models. The transfer of fish deterrent metabolites from food to opisthobranch has been recognized in many species.