  I present an informal discussion of various cases where two-dimensional
surface metal-insulator structural and charge-density-wave instabilities driven
by partly filled surface states have been advocated. These include
reconstructions of clean semiconductor surfaces and of W(100) and Mo(100), as
well as anomalies on the hydrogen-covered surfaces H/W(110) and H/Mo(110), and
possibly alkali-covered surfaces such as K/Cu(111). In addition I will also
discuss the opposite type of phenomena, namely surface insulator-metal
transitions, which can be argued to occur on $\alpha -$% Ga(001),
high-temperature Ge(111), and probably Be(0001).
