This contribution presents a brief overview of investigation and monitoring methods which may help to detect and localise deep-seated rock slides in the surroundings of reservoirs. The identification and localisation of critical slopes and ancient/pre-ex-isting rock slides in the early stage of a project (i.e. feasibility study) is essential to avoid endangering the planned infrastructure project and if necessary, adapting the project. The knowledge about deep-seated rock slides has increased over the decades. In particular, new insight was gained about rock slide geometry, kinematics, temporal deformation behaviour, hydrogeology and geomechanics. Major technical and methodical improvements have been made in recent years concerning the successful appli-cation of terrestrial and airborne based remote sensing tools to measure 2D/3D slope deformations on surface and to develop high-resolution digital terrain models for detailed geomorphological-geological mapping and geological-geometrical model desig.