Large-scale imaging in small brains

•Modern microscopy and genetic techniques enable large-scale imaging of small brains.•Imaging has generated insights into processes such as visual function, motor control and network dynamics.•Mapping out the behavioral repertoire of small model organisms will be crucial.

The dense connectivity in the brain means that one neuron's activity can influence many others. To observe this interconnected system comprehensively, an aspiration within neuroscience is to record from as many neurons as possible at the same time. There are two useful routes toward this goal: one is to expand the spatial extent of functional imaging techniques, and the second is to use animals with small brains. Here we review recent progress toward imaging many neurons and complete populations of identified neurons in small vertebrates and invertebrates.