A review is given of the experimental status of the interlayer coupling ene
rgy in the cuprates. A second c-ads plasmon is identified in the double lay
er compound Y123 for various dopings. The anomalous transport properties al
ong the c-direction and in the planar directions are compared to model calc
ulations based on strongly anisotropic scattering. An excellent description
of the optical data at optimal doping is obtained if an anomalously large
anisotropy of the scattering rate between cold spots and hot spots is assum
ed. This raises questions as to the physical meaning of these parameters.