The word “pain” was originally used to describe a human emotional negative experience. Pain is a clinical expression of tissue damage, an “unpleasant sensory and emotional experience” (International Association for the Study of Pain). While past research studies conducted on pain were focused on mammalians, current science is showing that also fish and shellfish species own functions and mechanisms potentially related to pain. Although there is insufficient research-based data required to answer questions related to the biology of pain in most of these species (i.e. existing receptors, pathways and specialized neurons to detect and recognize noxious stimuli), it may seem appropriate to give them the benefit of scientific doubt, at least. Here, the questioning triad and current approaches used in assessing pain in aquatic invertebrates shall be presented.