Bayesian Assurance is the unconditional probability that the trial will yield a positive result given a prior distribution for one or more planning parameters. The assurance equals the expectation for the power averaged over the prior distribution of the unknown parameter(s). The assurance provides a useful estimate of the likely utility of a clinical trial, creates an estimate for power which accounts for intrinsic pre-study uncertainty in our planning parameters and could provide an alternative method to frequentist power.