  We review various isotope effects in the high-T_c cuprate superconductors to
assess the role of the electron-phonon interaction in the basic physics of
these materials. Of particular interest are the unconventional isotope effects
on the supercarrier mass, on the charge-stripe formation temperature, on the
pseudogap formation temperature, on the EPR linewidth, on the spin-glass
freezing temperature, and on the antiferromagnetic ordering temperature. The
observed unconventional isotope effects strongly suggest that lattice
vibrations play an important role in the microscopic pairing mechanism of
high-temperature superconductivity.
