Animal behaviors and emotions are fine-tuned via signal transmission and communication between cells. The nervous system gives instructions and transmits signals to different parts of an organism to produce coordinated activities. Neurons are core components of the nervous system and they use various chemical messengers to communicate with one another. Neuropeptides (NPs), which are small chains of amino acids, represent the largest class of signaling molecules. Since NPs are directly involved in modulating many physiological processes, such as metabolism, reproduction, learning, memory, and social behaviors-there is a growing interest in studying their structures, functions, and distributions. This book aims to summarize advances in invertebrate NP studies concerning molecular diversities, distributions, and biological functions. In addition, the emerging mass spectrometric techniques have been discussed extensively as a powerful tool enabling accelerated invertebrate NP studies.