Neuropeptides are a diverse class of signalling molecules that act as long-lasting neurotransmitters in the nervous system and can evoke responses synaptically as well extrasynaptically. They coexist with other classical neurotransmitters and modulate the strength of synaptic signalling. Neuropeptide signalling alters membrane excitability, circuit plasticity, gene expression, and glial cell function, which collectively allow flexible tuning of behavioural outputs. As such, neuropeptidergic modulation is an attractive target for therapeutic interventions for disorders ranging across neuropsychiatric diseases, chronic/neuropathic pain, metabolic disorders, and anxiety and depression. However, the study of neuropeptides is challenging because of their molecular variability, structural diversity, receptor pleiotropy, non-synaptic action and low availability. In recent years, advances in genetic tools, electrophysiology, mass spectrometry, genetically-encoded activity sensors and chemo/optogenetic tools have permitted insights into the site-specific action of neuropeptides. These advances have galvanised neuropeptide research across scales from structure-function studies to the mechanisms encoding internal states and to the neurobiology of social interactions.