  We review some aspects of the isotope effect (IE) in superconductors. Our
focus is on the influence of factors not related to the pairing mechanism.
After summarizing the main results obtained for conventional superconductors,
we review the effect of magnetic impurities, the proximity effect and
non-adiabaticity on the value of the isotope coefficient (IC). We discuss the
isotope effect of $T_c$ and of the penetration depth $\delta$. The theory is
applied to conventional and high-$T_c$ superconductors. Experimental results
obtained for YBa$_2$Cu$_3$O$_{7-\delta}$ related materials (Zn and
Pr-substituted as well as oxygen-depleted systems) and for
La$_{2-x}$Sr$_x$CuO$_4$ are discussed.
