[en] Within this thesis a precise mass-prediction for the Higgs fields of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) is obtained with Feynman-diagrammatic methods. The results are studied numerically for sample scenarios that are in agreement with current New Physics searches at the LHC. Furthermore a comparison between the obtained results and different calculations is performed as a first step in order to obtain an estimation for the theoretical uncertainties of the Higgs-mass prediction in the NMSSM. The precise mass-prediction includes the full NMSSM one-loop corrections supplemented with the dominant and sub-dominant two-loop corrections within the Minimal Supersymmetric Standard Model (MSSM). These include contributions at the orders O(α_tα_s, α_bα_s, α_t"2, α_tα_b), as well as a resummation of leading and subleading logarithms from the top/scalar top sector. Higher-order corrections are essential for the NMSSM in order to provide a Higgs particle that is consistent with the available data, including the observed neutral, CP-even Higgs field with a mass of about 125 GeV. We explored the validity of the applied approximation at the two-loop level and found that it is reliable for a wide range of scenarios within the NMSSM. This is especially true for the mass of the observed (MS)SM-like Higgs field. The result of this work will be included in a future extension of the program FeynHiggs. We also compared our results with the program NMSSMCalc that also performs a Feynman-diagrammatic calculation of the Higgs-masses with a slightly different renormalization scheme. The comparison reveals that for the mass of the (MS)SM-like Higgs field the genuine NMSSM-effects induced by the choice of the renormalization scheme are by far minor compared to similar effects observed in the MSSM.