  Interstellar masers are unique probes of the environments in which they
arise. In studies of high-mass star formation their primary function has been
as signposts of these regions and they have been used as probes of the
kinematics and physical conditions in only a few sources. With a few notable
exceptions, we know relatively little about the evolutionary phase the
different maser species trace, nor their location with respect to other star
formation tracers. While detailed studies of a small number of maser regions
can reveal much about them, other information can only be obtained through
large, systematic searches. In particular, such surveys are vital in efforts to
determine an evolutionary sequence for the common maser species, and there is
growing evidence that methanol masers may trace an earlier phase than the other
common maser species of OH and water.
