We study higher-order corrections within the Higgs sectors of two extensions of the SM. First we deal with the Higgs sector of the 2HDM. The focus is on the development of a gauge-independent, process-independent and numerically stable renormalization scheme. Afterwards, we turn to the complex NMSSM and calculate the dominant two-loop corrections to the trilinear Higgs self-couplings. The calculation is performed in the gaugeless limit in the approximation of vanishing external momentum.