Understanding how the nervous system is formed and operates is a monumental task where the study of small, well-defined, neural circuits can provide insights into the principles of circuit organisation and neuronal processing. The spinal locomotor network operates as an autonomous metronome that creates a rhythmic output to muscles and coordinates left/right alteration during movement. Technical advances make zebrafish a unique model where the development, circuitry and function of cells within the locomotor circuitry can be investigated in vivo. The goal is to connect the formation of the circuit with the functionality of the circuit, both at the cellular and at the organism level. In vivo time-lapse imaging of fluorescent reporter lines will reveal how cells are generated and integrated within the locomotor circuitry. Free swimming zebrafish and detailed tail analysis combined with optogenetic stimulation and genetically encoded calcium-indicators will provide a direct link between signalling at a cellular level and functional output in the form of swimming behaviour. Detailed knowledge of circuit functionality is required to better understand how breakdown of networks relate to neural injuries and diseases.