The cosmic microwave background (CMB) radiation is the remnant heat left over from the Big Bang. This fossil relic has survived largely intact and it provides us with a unique probe of conditions in the early universe, long before any stars or galaxies had formed. I will describe what we have learned from painstaking measurements of the CMB, primarily from NASA's WMAP mission, including: evidence for the Big Bang itself; new measurements of the age, shape, and content of the universe; and new evidence that all structure in the universe emerged from microscopic quantum fluctuations generated during a period of cosmic inflation. I will conclude with a look ahead to new cosmological measurements in the works.