  I give a detailed introduction into the theoretical formalism for
epsilon'/epsilon, which measures direct CP-violation in K -> pi pi decays. The
current status of hadronic matrix elements and the strange quark mass is
discussed. Several possible explanations of the unexpectedly high experimental
results for epsilon'/epsilon are pointed out: A small strange quark mass, an
enhancement of the hadronic parameter B_6^(1/2) from the sigma resonance, an
underestimate of isospin breaking and possible new physics contributions in the
sdZ-vertex and the sd-gluon-vertex.
