  Theory of the isotope shift of the centroid energies of light few-electron
atoms is reviewed. Numerical results are presented for the isotope shift of the
$2^3P$-$2^3S$ and $2^1S$-$2^3S$ transition energies of $^3$He and $^4$He. By
comparing theoretical predictions for the isotope shift with the experimental
results, the difference of the squares of the nuclear charge radii of $^3$He
and $^4$He, $\delta R^2$, is determined with high accuracy.
