“Opisthobranch” mollusks, comprising the commonly known sea slugs and sea hares, present a wide variety of bioactive compounds protecting them against potential predators and competitors, and enhancing their ecological performance. Sea slug defenses include chemicals obtained directly from their prey, transformed dietary metabolites, or even de novo biosynthesized bioactive compounds. In this chapter, we describe what is known about the chemical ecology of marine slugs (“Opisthobranchs”) from a biological perspective and the molecules involved, focusing on the origins, bioactivities, and ecological roles in combination with other defensive strategies. Biogeographical and evolutionary aspects are also considered here. Often, chemical defenses are associated with warning (aposematic) colorations, allowing species to survive in exposed habitats, where predators learn to associate bright colorations with their bad taste. Many pigments possess bioactive properties themselves (e.g., alkaloids) while being part of photosynthetic systems, or may act as sunscreens protecting from UV light. Chemical defense allocation is particularly common in sea slugs, with extensive literature reporting bioactive products stored in exposed, vulnerable areas, such as the mantle, foot, gills, and rhinophores; within mucus or ink secretions; in specialized glands, like mantle dermal formations (MDFs); and also occasionally in eggs, embryos, and larval stages.