Detailed information on the gravitational effect of the Earth's topographic and isostatic masses is needed for various applications in geodesy and geophysics and can be calculated by gravity forward modeling (GFM). Within this thesis, the tesseroid-based Rock-Water-Ice (RWI) approach is developed, which provides an important contribution to state-of-the-art GFM. The basis of this approach is a rigorous separate modeling of the Earth’s rock, water, and ice masses with variable density values and a modified Airy-Heiskanen isostatic concept. For the numerical evaluation, optimized tesseroid formulas are elaborated that significantly reduce the computational demand. Besides a discussion and evaluation of the newly developed methods, applications in the context of the GOCE satellite mission and height system unification are presented. For the use in other research, several topographic-isostatic gravity field models are generated and made publicly available in terms of spherical harmonic coefficients.