The field of neurophotonics bridges optical physics and neuroscience, particularly on the creative use of optical technologies to probe brain function. Neurobiologists now rely heavily on neurophotonics, giving them the necessary tools to analyse the dynamics and principles of neural circuitry, especially on techniques to initiate and detect activity in networks of neurons, and to selectively excite sub-sets of neurons. Understanding how these networks of neurons in the brain process sensory inputs and shape motor outputs remains one of science’s great challenges.