  Extending for over 200 degrees across the sky, the Magellanic Stream together
with its Leading Arm is the most spectacular example of a gaseous stream in the
local Universe. The Stream is an interwoven tail of filaments trailing the
Magellanic Clouds as they orbit the Milky Way. Thought to be created by tidal
forces, ram pressure, and halo interactions, the Stream is a benchmark for
dynamical models of the Magellanic System, a case study for gas accretion and
dwarf-galaxy accretion onto galaxies, a probe of the outer halo, and the bearer
of more gas mass than all other Galactic high velocity clouds combined. If it
survives to reach the Galactic disk, it may maintain or even elevate the
Galactic star-formation rate. In this white paper, we emphasize the Stream's
importance for many areas of Galactic astronomy, summarize key unanswered
questions, and identify future observations and simulations needed to resolve
them. We stress the importance of ultraviolet, optical, and radio spectroscopy,
and the need for computational models that capture full particle and radiation
treatments within an MHD environment.
