Ground-based gamma ray astronomy studies some of the most extreme
environments in the universe, from neutron stars to supermassive black
holes.  Most of the detected sources of TeV gamma rays are episodic or
ephemeral by nature.  An instrument designed to continuously monitor the
entire overhead sky is needed to discover new sources and to observe TeV
counterparts of gamma ray bursts.  The Milagro all-sky telescope is the
first such instrument sensitive to gamma rays with energies below 1 TeV.
I will report on the science goals of the instrument, the design and
performance of Milagro, recent physics results, and ongoing and planned
upgrades to the instrument.