Theory of the helium isotope shift

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 $2^{3}P$-$2^{3}S$ and $2^{1}S$-$2^{3}S$ 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.