  According to a formula that was put forward many decades ago the ground state
energy per particle of an interacting, dilute Bose gas at density $\rho$ is
$2\pi\hbar^2\rho a/m$ to leading order in $\rho a^3\ll 1$, where $a$ is the
scattering length of the interaction potential and $m$ the particle mass. This
result, which is important for the theoretical description of current
experiments on Bose-Einstein condensation, has recently been established
rigorously for the first time. We give here an account of the proof that
applies to nonnegative, spherically symmetric potentials decreasing faster than
$1/r^3$ at infinity.
