The residual radiation from the Big Bang is observable as the cosmic microwave background-a near-uniform sea of photons with an average temperature of 2.7 K. Small variations in the temperature of the microwave background reflect the conditions that prevailed in the early Universe. Analysis of the physical mechanisms by which these anisotropies formed shows how they may be used to constrain fundamental cosmological parameters and provide insight into the origin of large-scale structure in the Universe.