  In recent years, a number of extragalactic massive star clusters that are
still deeply embedded in their birth material have been discovered. These
objects represent the youngest stage of massive star cluster evolution yet
observed, and the most massive and dense of these may be proto globular
clusters. Their properties appear to be similar to those of ultracompact HII
regions in the Galaxy, but scaled up in total mass and luminosity. In many
cases, these clusters are only visible at mid-IR to radio wavelengths, and they
have typically been detected as ``inverted'' spectrum radio sources. However,
the set of existing observations is anemic, and our current physical model for
these natal clusters in simplistic. This article will overview what we think we
know about these objects based on existing observations and outline some of the
most significant gaps in our current understanding.

Astrophysics