Finally, it is widely held that long-duration GRBs are highly collimated explosions: bi-conical jets with opening angles ranging from several to several tens of degrees \cite{Sari_1999}. At early times, when the Lorentz factor of the outflow is very high, observers will not realize that they are viewing a collimated outflow due to relativistic beaming. However, as the jet sweeps up material and slows down, observers will be able to see larger and larger fractions of the jet. Eventually, when the Lorentz factor is equal to the inverse of the jet opening angle, the jet will begin to spread laterally and observers will notice "missing" emission from wider angles. This transition is known as a jet break, and results in an achromatic steepening of the afterglow light curve.