This dissertation studies a coupled flow and deformation model for saturated and non-saturated soils, based on Biot`s theory but taking into account the nonlinear material behavior of soil. In the non-saturated condition the fluid flow can be considered either as a two-phase or an one-phase problem. In the two-phase case, the model calculates the pressure of air and water simultaneously, thus permitting the evaluation of the pore suction values and the degree of saturation. Once known, the degree of saturation is used for the determination of the relative permeability of the soil considering the flow of water and air, solving a problem that is nonlinear with respect to permeability. In the one-phase flow, the pressure of air is kept constant and equal to the atmospheric pressure. The stress-strain nonlinear behavior of non-saturated soils is modeled through the basic Barcelona model (BBM model) based on plasticity theory and concepts from the critical state behavior of soils. This model may be understood as an extension of the modified Cam Clay model frequently used for representation of the mechanical behavior of saturated soils. The suction influence on the shear resistance of non-saturated soils is examined through some numerical applications. In this work, the material nonlinearity of dry or saturated soils is also simulated through the Lade & Kim elastoplastic model.