  The energies of $^{3}H$, $^{3}He$, and $^{4}He$ ground states, the
${\frac{3}{2}}^{-}$ and ${\frac{1}{2}}^{-}$ scattering states of $^{5}He$, the
ground states of $^{6}He$, $^{6}Li$, and $^{6}Be$ and the $3^{+}$ and $0^{+}$
excited states of $^{6}Li$ have been accurately calculated with the Green's
function Monte Carlo method using realistic models of two- and three-nucleon
interactions. The splitting of the $A=3$ isospin $T=\frac{1}{2}$ and $A=6$
isospin $T=1$, $J^{\pi} = 0^{+}$ multiplets is also studied. The observed
energies and radii are generally well reproduced, however, some definite
differences between theory and experiment can be identified.
