Functional multineuron calcium imaging (fMCI) is a large-scale optical recording technique that monitors the spatiotemporal pattern of action potentials, all at once, from large neuron populations. fMCI has unique advantages, including: (i) simultaneous recording from >1000 neurons in a wide area, (ii) single-cell resolution, (iii) identifiable location of neurons and (iv) detection of non-active neurons during the observation period. We review herein the principle, history, utility and limitations of fMCI.