Until the twentieth century, principles of symmetry played little explicit role in theoretical physics. Conservation laws, especially those of energy and momentum, were considered to be of fundamental importance. But these were regarded as consequences of the dynamical laws of nature, rather than as consequences of the symmetries that underlay these laws. Maxwell's equations, formulated in 1865, embodied both Lorentz invariance and gauge invariance. But these symmetries of electrodynamics were not fully appreciated for 40 years or more.