Deformation and localization analysis is a crucial issue and has thus been intensively investigated in the last decades. However, in contrast to solid mechanical problems, geotechnical applications do not only concern a single solid material, the soil, but they also affect the pore-fluids, water and air, and, consequently, the coupling of the solid deformation with the pore-fluid flow. As a result, both the deformation and the localization analysis must be applied to a triphasic material consisting of the soil skeleton, the pore-water and the pore-gas, which, in geotechnical engineering, is known as unsaturated or partially saturated soil. Based on a continuum mechanical approach, unsaturated soil can be described within the well-founded framework of the Theory of Porous Media (TPM), thus including saturated soil (solid matrix and pore-water) as well as empty soil (solid matrix and pore-gas) as special cases.It is the goal of the present contribution to investigate the deformation and the localization behavior of unsaturated soil and to exhibit the influence of the solid-fluid coupling on the localization analysis. In the framework of a triphasic formulation, unsaturated soil is considered as a materially incompressible elasto-plastic or elasto-viscoplastic skeleton saturated by two viscous pore-fluids, a materially incompressible pore-liquid and a materially compressible pore-gas. Assuming quasi-static situations, the numerical computations proceed from weak formulations of the momentum balance of the overall triphasic material together with the mass balance equations of the pore-fluids and Darcy-like relations for the seepage velocities. As a result, a system of strongly coupled differential-algebraic equations (DAE) occurs, which is solved by use of the FE tool PANDAS. In particular, various initial boundary-value problems are treated on the basis of time- and space-adaptive methods, thus demonstrating the efficiency of the overall formulation. Furthermore, the influence of the pore-gas constituent on the material behavior of partially saturated soil is studied with respect to fluid-flow simulations or embankment and slope failure problems.