  High mobility two-dimensional electron gases (2DEGs) underpin today's silicon
based devices and are of fundamental importance for the emerging field of oxide
electronics. Such 2DEGs are usually created by engineering band offsets and
charge transfer at heterointerfaces. However, in 2011 it was shown that highly
itinerant 2DEGs can also be induced at bare surfaces of different transition
metal oxides where they are far more accessible to high resolution angle
resolved photoemission (ARPES) experiments. Here we review work from this
nascent field which has led to a systematic understanding of the subband
structure arising from quantum confinement of highly anisotropic transition
metal d-states along different crystallographic directions. We further discuss
the role of different surface preparations and the origin of surface 2DEGs, the
understanding of which has permitted control over 2DEG carrier densities.
Finally, we discuss signatures of strong many-body interactions and how
spectroscopic data from surface 2DEGs may be related to the transport
properties of interface 2DEGs in the same host materials.
