  For more seventy years physicists have appreciated that Nature's vacuum is
far from empty. The discovery of the Lamb shift in Hydrogen provided dramatic
verification of the reality of the quantum vacuum. The advent of gauge theories
has led us to believe that the physics of the vacuum is even richer, with the
possibility of instantons, vacuum phase transitions, vacuum defects (monopoles,
domain walls, cosmic strings and nontopological solitons), vacuum energy, and
degenerate vacua states (with different local realizations of the laws of
physics). Cosmology offers a unique laboratory for exploring the ``physics of
nothing.'' In this lecture I focus on the implications of vacuum energy for
cosmology -- in particular, inflation -- and discuss the flood of observations
that are testing the inflationary paradigm and in process probing the physics
of nothing. I also discuss the possibility that today vacuum energy plays a
dynamically important role (as a cosmological constant).
