Landslides are the most common type of geohazards in mountainous areas. They have tremendous socio-economic impact, especially in developing countries (Brabb & Harrod, 1989; Wang, 1992; Schuster, 1994). In a broad sense, landslide is a general term applied to all modes of downslope movement of a mass of soil and rock material, such as slides, debris flows, rockfalls, topples, etc. (Varnes, 1978; Hutchinson, 1988; Cruden & Varnes, 1996). In a narrow sense, landslide usually refers to the slide mode of soil and/or rock mass movement, in which there is a distinct surface or zone at the contact between the moving mass and the underlying stable materials (Zaruba & Mencl, 1982; Wen & Aydin, 2003; Cornforth, 2005). In this book, the term landslide is used only in this narrow sense.