================================================================== Sign Problems in Quantum Monte Carlo and Meron Cluster Algorithms ================================================================== Ab-initio studies of strongly correlated systems at finite densities and at real-times are hindered by the infamous sign problem. In this discussion, we analyze the problem for fermionic many-body quantum systems (quantum field theories on the lattice) and discuss criteria for solving such a sign problem. Meron cluster algorithms are then introduced and discussed taking the example of the fermionic t-V model. The algorithms can be generalized to simulate Abelian gauge theories (realized as quantum link models) coupled to fermions in any space-time dimension. References: 1. Computational complexity and fundamental limitations to fermionic quantum Monte Carlo simulations, Matthias Troyer, Uwe-Jens Wiese, Phys.Rev.Lett. 94 (2005) 170201. 2. Meron cluster approach to systems of strongly correlated electrons, S. Chandrasekharan, J. Cox, J.C. Osborn, U-.J. Wiese, Nucl.Phys.B 673 (2003) 405-436