April 15, 1874, was a landmark day in art, marking the opening of the first exhibition by a group who would later become known as Impressionists. And it may be easy to forget, when looking at their paintings of middle-class leisure, but the Impressionists were radicals. They broke from academic tradition, painting modern life rather than historical or mythological subjects, and were more concerned with capturing ephemeral moments than using illusionist tricks. As Impressionism turns 150, learn more about the artists and work that defined an era.