Amenable unitary representations of a locally compact group, G, are studied in terms of associated coefficient subspaces of the Fourier-Stieltjes algebra B(G), and in terms of the existence of invariant and multiplicative states on associated von Neumann and C * -algebras. We introduce Fourier algebras and reduced Fourier-Stieltjes algebras associated to arbitrary representations, and study amenable representations in relation to these algebras.