Successful advice on foundation systems requires a sound subsoil model that already takes into account the requirements of the structure and includes the key soil mechanics parameters. For an economical design, the structure-soil interaction must also be mapped as accurately as possible. This requires numerical calculations (e.g. 3D FEM), which may also have to take dynamic effects and dynamic subsoil properties into account. Large-scale field tests, e.g. on piles, provide valuable information on the actual load-deformation behavior and generally lead to a more economical design. In the case of existing foundations, the structural condition must be recorded and documented as accurately as possible.