The aim of this chapter is to describe the usefulness of relatively simple model systems to study the roles played by neuropeptides in the mediation of adaptive and homeostatic behaviors. In particular, we will discuss some of the experimental approaches to exploit the unique characteristics of gastropod molluscan nervous systems. The recent discoveries and initial characterizations of neuroactive peptides, peptidergic neurons, and systems of such neurons have fundamentally altered our classical views regarding the transmission of neural information. The view that transmission of chemical signals within the brain largely involves only a very few neural transmitters with one transmitter per neuron and transmitter release only at anatomically stereotyped synapses no longer is tenable.