  Smooth manifolds of G_2 holonomy, used to compactify M-theory to four
dimensions, give only abelian gauge groups without charged matter multiplets.
But singular G_2-manifolds can give abelian or nonabelian gauge groups with
chiral fermions. We describe the mechanism of anomaly cancellation in these
models, which depends upon anomaly inflow from the bulk. We also compare the
anomaly predictions to what has been learned by more explicit arguments in some
special cases.
