  We present a general strategy to solve the notorious fermion sign problem
using cluster algorithms. The method applies to various systems in the Hubbard
model family as well as to relativistic fermions. Here it is illustrated for
non-relativistic lattice fermions. A configuration of fermion world-lines is
decomposed into clusters that contribute independently to the fermion
permutation sign. A cluster whose flip changes the sign is referred to as a
meron. Configurations containing meron-clusters contribute 0 to the path
integral, while all other configurations contribute 1. The cluster
representation describes the partition function as a gas of clusters in the
zero-meron sector.
