The genus Deroceras Rafinesque, 1820 (the largest genus of terrestrial slugs) shows a high diversity of penis morphologies and mating behaviors. The function of most of the appending external and internal penial structures, some of them truly bizarre, is largely unknown. This paper reviews mating behavior and reproduction, based on data on 16 species from the literature and from unpublished observations. I analyze patterns common to all Deroceras species and differences among species. The general mating pattern consists of a long courtship with mutual stroking with a sarcobelum, a sudden penis eversion, and external sperm exchange (copulation). I distinguish also precourtship and withdrawal phases. Sperm exchange is usually very quick but, in a few species, occupies a considerable proportion of the total mating duration. Mutual sperm exchange is the rule. Species differences involve the durations of certain mating phases, presence and nature of initial trail following, nature and intensity of stroking (including the degree of contact with the sarcobelum), aggressiveness of courtship behavior, and the timing of the penial gland eversion. I hypothesize that the radiation of mating behaviors and associated structures has been driven by an arms race resulting from conflicting interests of mating partners over sperm donation and use. This could also have increased the rate of speciation in Deroceras. There are indications of the presence of sperm competition and conflicting interests between mating partners: individuals mate repeatedly, can store and digest sperm, and simultaneously use sperm from different mating partners for fertilization. Some details of mating behavior also indicate conflict. The timing of the penial gland eversion after sperm exchange suggests a manipulation akin to the role of love darts in helicid snails. Finally, some recommendations for studying mating behavior in Deroceras are given.