Pebble-bed reactors belong to the class of High-Temperature Reactors (HTRs), one of the six Gen-IV nuclear system candidates. A distinct feature of pebble-bed reactors is that their core has a ‘double heterogeneity’ structure, with TRISO fuel particles randomly distributed withing a graphite matrix in the form of spherical pebbles, themselves randomly distributed within the core, and possibly intermixed with graphite pebbles (without fuel particles) acting as moderator. In this MSc thesis we address the problem of modeling the transport and multiplication of neutrons within pebble-bed reactors using Monte Carlo simulation. In order to achieve our goals, we consider two aspects: sampling the random geometries typical of such class of reactors, and then solving the eigenvalue problem describing neutron propagation in each realization of the underlying reactor model.