  Quantum field theories (QFTs) at finite densities of matter generically
involve complex actions. Standard Monte-Carlo simulations based upon importance
sampling, which have been producing quantitative first principle results in
particle physics for almost fourty years, cannot be applied in this case.
Various strategies to overcome this so-called Sign Problem or Complex Action
Problem were proposed during the last thirty years. We here review the sign
problem in lattice field theories, focussing on two more recent methods:
Dualization to world-line type of representations and the density-of-states
approach.
