The largest earthquakes occur in subduction zone environments, and most of them involve slip on the megathrust. But the slip behavior of the megathrust is highly variable along strike, downdip, and also in time. In this talk, I summarize more than 20 years of work on slip on the Alaska megathrust throughout the earthquake cycle,  highlighting the variations in space and time,   and the additional factors that need to be understood in order to estimate the slip behavior.  While the root causes of  along-strike changes in slip remain uncertain,  in some cases the along-strike transitions can be correlated with inferred or measured changes in the  properties  of  the plate interface.