The human visual system is a network of neural components that combine to create our perception of the world and guide our behavior. Deciphering the computational principles of this system is an important scientific challenge. We review measurements of these components, from the retinal encoding to cortical circuitry, and from molecules to circuits, focusing on measurements that are relevant to visual processing. We then delve into principles proposed to explain how this diverse collection of visual components enables us to interpret our surroundings.