  These are the lecture notes of an introductory course on ordinal analysis.
Our selection of topics is guided by the aim to give a complete and direct
proof of a mathematical independence result: Kruskal's theorem for binary trees
is unprovable in conservative extensions of Peano arithmetic (note that much
stronger results of this type are due to Harvey Friedman). Concerning
prerequisites, we assume a solid introduction to mathematical logic but no
specialized knowledge of proof theory. The material in these notes is intended
for 12 lectures and 6 exercise sessions of 90 minutes each.
