  The yrast states of transitional nuclei are described as quadrupole waves
running over the nuclear surface, which we call tidal waves. In contrast to a
rotor, which generates angular momentum by increasing the angular velocity at
approximately constant deformation, a tidal wave generates angular momentum by
increasing the deformation at approximately constant angular velocity. The
properties of the tidal waves are calculated by means of the cranking model in
a microscopic way. The calculated energies and E2 transition probabilities of
the yrast states in the transitional nuclides with $Z$= 44, 46, 48 and $N=56,
58, ..., 66$ reproduce the experiment in detail. The nonlinear response of the
nucleonic orbitals results in a strong coupling between shape and single
particle degrees of freedom.
