  This article reviews observations and models of the diffuse ionized gas that
permeates the disk and halo of our Galaxy and others. It was inspired by a
series of invited talks presented during an afternoon scientific session of the
65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory
in August 2004. This review is in recognition of Carl's long standing interest
in and advocacy for studies of the ionized as well as the neutral components of
the interstellar medium.
