Investigation results of the influence of weathering process on the residual shear strength and durability of fine grained lithological flysch components in the area of north Istria, Rječina Valley and Vinodol Valley are presented in this doctoral theises. Physical and mechanical properties of incompetent, fine grained flysch lithoilogcal members significantly change due to weathering. Landslide occurrences in flysch deposits are frequent and slip surface is formed at the contact of surficial deposits and flysch rock mass or inside the weathered flysch rock mass. Rock mass defragmentation at the slip surface occurs due to sliding and shear strength decreases from peak to residual values thorugh occured displacements. These residual strength values of the slip surface material are competent for the dormant landslide reactivation along the same slip surface. Weathering effect on the residual shear strength values of fine grained lithological flysch components are investigated by laboratory tests on samples of different weathering grades in direct shear and ring shear apparatus. Also uniaxial compressive strenth and durability of fine grained lithological flysch rock mass samples with different weathering grades are investigated. Mineralogical composition of the same fine grained lithological flysch components of different weathering grades is determined. Numerical simulation of the Krbavčići II landslide in north Istria and Valići landslide in the Rječina Valley reactivation are conducted with residual strength parameters of different weathering grades of fine grained lithological flysch components. It is proved that weathering process has significant effect on the residual shear strength decrease of fine grained lithological flysch components and that weathering of those materials causes the dormant landslide reactivation in the flysch rock mass.