I'd have to disagree with PJ, as creating glass with Add, instead of 'Add Special' can lead to some ugly BSP holes. And they might not be there the first time you try it but sneak in later, at least that's how i came across it.
The way to create glass that has worked for me is this:
1) Create your window opening as PJ described it
2) create your sheet brush to fit the opening
3) build it using 'add special' and using the preset for glass (i think you need to check 2-sided as well)
4) add your glass texture. Now you have glass you can walk and shoot through
5) create an cube brush (about 2 units thick that's just slightly smaller than your window and put it near the window so that it does not touch the walls or your glass. Do a normal 'add' then select all surfaces and make the brush invisible using the surface properties dialog.
--ether
http://www.remixradio.com/ut/
[This message has been edited by ether (edited 03-23-2000).]