From rainbows to the mechanics of the human eye, light has fascinated scientists for millennia. Today, the latest optical technologies — from lasers to solar cells — harness light to further physics and serve societies’ needs. To focus on these advances the United Nations has designated 2015 the International Year of Light and Light-based Technologies. It marks anniversaries including one of the first scientific accounts of optics published by the Islamic scholar Ibn al-Haytham in 1015; August Fresnel’s proposal in 1815 that light is a wave; James Clerk Maxwell’s 1865 electromagnetic theory of light; Albert Einstein’s 1915 general theory of relativity; and in 1965 the discovery of the cosmic microwave background radiation and the development of optical fibres for communication. Our special issue explores how scientists are pushing the properties of light to new extremes.