The best thermoelectric materials are believed to be heavily doped semiconductors. The presence of a bandgap is assumed to be essential to achieve large thermoelectric power factor and figure of merit. In this talk, we study several semimetals including HgTe as an example semimetal with competitive thermoelectric properties. We show theoretically and experimentally that semimetals can achieve large thermoelectric figure of merit. We employ ab initio calculations with hybrid exchange-correlation functional to accurately describe the electronic band structure of several semimetals in conjunction with the Boltzmann Transport theory to investigate their electronic transport properties. We show that semimetals can have Seebeck coefficient values close to those of semiconductors. Their electrical conductivity values are higher than semiconductors and they can have similar thermal conductivity values. We calculate the lattice thermal conductivity of HgTe using first principles calculations and evaluate its overall figure of merit. Finally, we prepare semi-metallic HgTe samples and we characterize its transport properties. We show that our theoretical calculations agree well with the experimental data.