Terrestrial slugs and snails are particularly favorable subjects for studies of comparative cognition. Chemosensation is their dominant distance sense for locating food, mates, predators, and nest sites. A variety of conditioning phenomena have been demonstrated using odors as conditioned stimuli, including higher order conditioning such as second-order conditioning and blocking. Learning has been evaluated by measuring local reflexes and whole body orientation to odors. Behaviors with learned components involve homeostatic mechanisms for water, temperature, nutrition, and circadian activity. Cellular substrates and neural correlates for plasticity in odor processing have focused on a unique brain region, the procerebral lobe, which is necessary and sufficient for learning about odor stimuli. The rich set of learning phenomena displayed by terrestrial slugs and snails emphasize the importance of seeking evidence for complex cognitive tasks by asking experimental questions appropriate to the Ümwelt of the animal. In general, invertebrates can implement most vertebrate learned logic operations.