This chapter discusses the mathematical foundation of the signed-particle method used to solve the Wigner-Boltzmann equation using Monte Carlo techniques. The integral formulation of the WBE is introduced. The integral form is developed into a Neumann series, which is evaluated using Monte Carlo integration. Finally, the basic building blocks and architecture of the algorithm implemented in the Wigner Ensemble Monte Carlo (WEMC) simulator – as part of the open source project ViennaWD [102] – are presented.