Coelenterates include the invertebrate jellyfish and their relatives, and were the first group of animals to have a recognizable nervous system. This symposium on their neurobiology emphasizes neurotransmitters, neural pathways, and sensory receptors, in the context of mechanisms underlying behavior. It presents and integrates recent discoveries from modern neuroscience, genomics and phylogenetics about an evolutionarily crucially important group of animals. This Symposium is part of the International Conference on Coelenterate Biology (which last met in 1995), which attracts investigators from many different countries, and the this is the first time this meeting has been held in the USA. This symposium will have scientific impact on understanding evolution of nervous systems from some of the simplest forms of neuronal nets and the first sensory organs, and will have impact beyond neuroscience to a better understanding of the evolutionary importance of the coelenterates. Additionally, the conference attendance of undergraduate and graduate students, and postdoctoral workers and junior faculty will insure several opportunities for promoting interactions with established laboratories that will have impacts on the careers of young investigators.