  The quantum mechanical mass of 't Hooft-Polyakov monopoles in the
four-dimensional Georgi-Glashow is calculated non-perturbatively using lattice
Monte Carlo simulations. This is done by imposing twisted boundary conditions
that ensure there is one unit of magnetic charge on the lattice, and measuring
the free energy difference between this ensemble and the vacuum. In the
weak-coupling limit, the results can be used to determine the quantum
correction to the classical mass, once renormalisation of couplings is taken
properly into account. The methods can also be used to study the masses at
strong coupling, i.e., near the critical point, where there are hints of a
possible electric-magnetic duality.
