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Published online by Cambridge University Press: 26 May 2016
In recent years, a number of extragalactic massive stars clusters have been detected which are still deeply embedded in their birth material In many cases, these clusters are only visible in the mid-IR to radio regimes, and have typically been detected as ‘inverted’ spectrum radio sources. These clusters appear to have properties which are analogous to those of ultracompact H ii (UCH ii) regions in the Galaxy, albeit vastly scaled up in total mass and luminosity. The ultra-young clusters appear to form with a continuum of masses, and the most massive are consistent with being the progenitors of globular clusters. Here I will overview the properties of the currently known sample of ultradense H ii (UDH ii) regions and examine their relation to Galactic UCH ii regions.