The threat of a great (M ∼9) earthquake along the Cascadia subduction zone is evidenced by both paleoseismology data and current strain accumulation along the fault. On the basis of recent information on the characteristics of this subduction system, we estimate the conditional probabilities of a great earthquake occurring within the next 50 years and their variabilities. The most important variation is associated with the existence of episodic slow slip on the deep portion of the subduction interface. We show that these events modulate the conditional probability dramatically over their ∼14-month cycle. During the 2-week slow-slip events, the weekly probability of a great earthquake is about 30 to 100 times as high as it is during any week of the rest of the year. Near-term probabilities also vary significantly with the assumed distribution of earthquake recurrence intervals. Under a reference scenario of unimodal distribution of recurrence intervals (about 500-600 years), the 50-year conditional probability is low (0-12% at a 95% confidence interval). However, under a tantalizing bimodal distribution hypothesis, this probability could be either four times as high (6-45%) or four times as low (less than 1%), depending on whether the current interval is short (∼350 years) or long (∼850 years).