Lahars, an Indonesian term to indicate volcanic debris flows and hyperconcentrated flows with various amount of volcanic solid contents and represent one of the most significant hazard for people living in volcanic areas. The main goal of this thesis is to investigate the initiation of lahars triggered by rain in volcanic areas during both long-lasting and short-lived eruptions combining field observations, geotechnical tests and numerical modelling applied to two case studies: the 2011 eruption of Cordón Caulle volcano (Chile) and La Fossa volcano, Vulcano island (Italy). An application of two slope stability and one erosion models (TRIGRS, SHALSTAB and LISEM) for rainfall-triggered lahars is presented. The rain-triggered lahars susceptibility maps presented in this thesis provide an innovative approach to the evaluation of the volume of pyroclastic material potentially remobilized by rain during and after explosive eruptions that is important to include in comprehensive hazard assessments of lahar inundation.