We are interested in understanding how the nervous system generates learning. One way to address this question is to identify and characterize conserved features in the underlying mechanisms. To accomplish this goal, we use Caenorhabditis elegans in our studies, because its nervous system is small and has been well characterized. Meanwhile, various genetic, molecular and imaging methods can be readily applied to the experiment with the worm. Our research has been mainly focused on olfactory learning. In the past years, our work has yielded new understanding on the function of the underlying neural circuits. For example, we have mapped a neuronal network that encodes both the naive and learned olfactory preferences. We have identified and characterized compartmentalized activity in the key neuron of the learning circuit that is similar to corollary discharge. We have also characterized circuit function of neuromodulators that regulate learning through ligand-receptor interactions.