Homebrew PCBs

Hi guys,

Last night I was thinking about 3 and 4 layer PCBS and can't seem to 
figure out how they do them.

Single sided - easy, we all do that.
Double sided - easy, we do that as well with some alignment issues.

But once you get beyond that, I realize a different manufacturing 
process has to be done to get the layers.  How do they do it?

Chris