The accuracy to which the vertical structure of tidal currents can be predicted is examined. Theoretical models for current structure are developed employing (a) a constant eddy viscosityE = ϵand (b) an eddy viscosity varying linearly with height above the sea bedz;E(z)=βz. By requiring these models to satisfy the commonly accepted quadratic friction law, the condition ϵ>½k is deduced wherekis the bed friction coefficient,Wa representative velocity andDthe depth.