Common two-phase flows are formed from a mixture of gas and liquid flowing through
a vertical pipe. These flows admit a variety of instabilities, and the one of 
particular concern here is that which leads to voidage waves in bubbly flows. 
Waves of this sort can be observed propagating downwards in a glass of Guinness, 
as well as other similar types of beers. We study a two-phase model for the flow, 
and show that waves can arise through an instability analogous to that which 
produces roll waves in shallow water flow.