_Background. Silver nanoparticles (AgNPs) act as antimicrobials by releasing monovalent silver (Ag⁺) and are increasingly used in consumer products, thus elevating exposures in human and environmental populations. Materials and methods. We evaluated Ag⁺ in a standard model of neuronal cell replication and differentiation, and then determined whether there were similar effects of the ion in vivo using zebrafish. Next, we compared Ag⁺ and AgNP exposures in the same two models and incorporated the effects of particle coating, size and composition. Conclusions. This work is the first to show that both Ag⁺ and AgNPs are developmental neurotoxicants in vitro and in vivo. Moreover, although both the soluble ion and the particles impair measures of neurodevelopment, the outcomes and underlying mechanisms of each toxicant are often wholly distinct. Superimposed on the dichotomies between Ag⁺ and AgNP exposures are clear effects of particle coating, size and composition that will necessitate evaluation of individual AgNP types when considering potential environmental and human health effects. The results presented here provide hazard identification that can help isolate the models and endpoints necessary for developing a risk assessment framework for the growing use of AgNPs.