The mechanisms causing the reduction in lattice thermal conductivity in highly P- and B-doped Si are looked into in detail. Scattering rates of phonons by point defects, as well as by electrons, are calculated from first principles. Lattice thermal conductivities are calculated considering these scattering mechanisms both individually and together. It is found that at low carrier concentrations and temperatures phonon scattering by electrons is dominant and can reproduce the experimental thermal conductivity reduction. However, at higher doping concentrations the scattering rates of phonons by point defects dominate the ones by electrons except for the lowest phonon frequencies. Consequently, phonon scattering by point defects contributes substantially to the thermal conductivity reduction in Si at defect concentrations above 10 19 cm − 3 even at room temperature. Only when, phonon scattering by both point defects and electrons are taken into account, excellent agreement is obtained with the experimental values at all temperatures. an ab-initio study of n and p -doped Si and showed that, EPS at a carrier concentration of p ≈ 10 21 cm − 3 can result in a ≈ 45% reduction in κ (cid:96) at room temperature. The calculations reproduce how κ (cid:96) is lower in p doped samples than in n -doped ones, in agreement with the experiments. However, they do not capture the magnitude of the reduction observed in B-doped p -type single-crystal Si, which at a doping level of 5 × 10 20 cm − 3 amounts to more than 70%. Successful prediction of thermal-conductivity of highly-doped Si, revealing the importance of phonon scattering by electrons as well as point defects.