  The Cosmic Microwave Background (CMB) is the oldest light in the universe. It
is seen today as black body radiation at a near-uniform temperature of 2.73K
covering the entire sky. This radiation field is not perfectly uniform, but
includes within it temperature anisotropies of order delta(T)/T ~ 10E-5.
Physical processes in the early universe have left their fingerprints in these
CMB anisotropies, which later grew to become the galaxies and large scale
structure we see today. CMB anisotropy observations are thus a key tool for
cosmology. The Planck Mission was the European Space Agency's (ESA) probe of
the CMB. Its unique design allowed CMB anisotropies to be measured to greater
precision over a wider range of scales than ever before. This article provides
an introduction to the Planck Mission, including its goals and motivation, its
instrumentation and technology, the physics of the CMB, how the contaminating
astrophysical foregrounds were overcome, and the key cosmological results that
this mission has so far produced.
