The strength and deformation characteristics of sedimentary soft rock evaluated in relation to several large-scale research and construction projects in Japan are reviewed. The elastic stiffness at small strains,non-linearity due to strain and pressure level,and viscous properties are described. The elastic stiffness from triaxial tests using high-quality core samples while measuring stresses and strains accurately is basically the same as the corresponding value from field shear wave velocity. It is necessary to take into account the dependency of stiffness on shear strain and pressure level,which could be evaluated by relevant laboratory stress-strain tests while referring to results from relevant field loading tests. Loading rate effects due to the material viscous properties could be simulated by a non-linear three-component model.