In European seismic areas timber structures are found as building frames, in combination with masonry infills, in bridges, but most frequently in roof structures and floor slabs of traditional buildings. In the seismic design of new timber structures, members are elastic and ductility resources are concentrated in the joints. Similarly, seismic strengthening of existing structures should provide a well-defined and simple path to seismic forces, maintain timber members elastic, and develop as much as possible the post-elastic behaviour of joints. Provisions must be adopted to avoid sudden loss of capacity and brittle failure, and to foster ductility. Different criteria for seismic strengthening of floor slabs and of carpentry joints are presented.