The MSSM Higgs Sector at O(alpha_b alpha_s) and Beyond

We evaluate O(alpha_b alpha_s) corrections in the MSSM Higgs boson sector, including an analysis of the renormalization in the bottom/scalar bottom sector. For mu < 0 the genuine two-loop corrections to the mass of the lightest Higgs boson mass can amount up to 3 GeV. Different renormalization schemes are applied and numerically compared. Likewise the residual dependence on the renormalization scale is investigated. This allows to determine the remaining theoretical uncertainties from unknown higher-order corrections at O(alpha_b alpha_s^2) for different regions of the MSSM parameter space.