In this paper we focus on the quasi-static modeling of SOI-MOSFETs, First we discuss general considerations of charge based modeling in semiconductor devices. Advantages and disadvantages of charge based formulations will be discussed, Next a compact SOI model which has bern developed by our group will be introduced, This model includes the description of the smooth transition between fully depleted and partially depleted operating modes. Utilizing this model a charge based formulation of the inner fringing capacitance in an SOI-MOSFET will be presented. Results of this new model will be compared to those gained by numerical device simulation.