  We investigate the effect that the structure of GRB jets has on the afterglow
light curves for observers located at different viewing angles, $\theta_{obs}$,
from the jet symmetry axis. The largest uncertainty in the jet dynamics is the
degree of lateral energy transfer. Thus, we use two simple models, that make
opposite and extreme assumptions for this point, and calculate the light curves
for an external density that is either homogeneous, or decreases as the square
of the distance from the source. The Lorentz factor and kinetic energy per unit
solid angle are initially taken to be power laws of the angle from the jet
axis. We perform a qualitative comparison between the resulting light curves
and afterglow observations. This constrains the jet structure, and poses
problems for a `universal' jet model, where all GRB jets are assumed to be
intrinsically identical, and differ only by our viewing angle, $\theta_{obs}$.
