Book article:
Samuel R. Buss.
"An Introduction to Proof Theory"
in Handbook of Proof Theory, edited
by S. R. Buss.
Elsevier, Amsterdam, 1998, pp 1-78.
Download article: postscript or PDF.
Table of contents: This is an introduction to proof complexity.
Errata.
1. Theorem 2.5.3 (the generalized Herbrand Theorem) on page 52 is correct as stated, but has an error in its proof. I am grateful to Richard McKinley for discovering this, and also providing a corrected proof. A corrected proof is available in the manuscript "A sequent calculus demonstration of Herbrand's Theorem" by Richard McKinley.