Fluid (water) flowing through porous soils and rocks under and in the vicinity of loaded engineering structures causes coupled effects exhibited by interacting deformation and fluid (pore) water pressures. The resulting fluid pressures cause changes in the effects of mechanical loading, and thereby influence the stability of soil-structure interaction systems. For some problems, we could assume that the skeleton of the geologic medium experiences no deformations. The water flowing through the pores of such rigid medium, which is often called seepage, causes forces or pressures that are to be evaluated for the analysis and design of geotechnical structures. In this chapter, we consider seepage and its relation to the stability of geotechnical structures.