Or, in the original German
, ``Über formal unentscheidbare
Sätze der Principia Mathematica
und verwandter Systeme''. The theorem proved therein is also known as Gödel's Incompleteness Theorem
The proof of incompleteness assumes, of course, that said formal system is consistent.
Yes, outside links are naughty, but see http://www.ddc.net/ygg/etext/godel/index.htm