The conventional approach is to apply standard estimating procedures to achieve a single deterministic cost number, to which a contingency is applied for the purposes of arriving at the final budget figure. This approach is appropriate for many well understood project scenarios. However, deepwater field abandonment, well plugging and abandonment (P&A) in particular, presents a much higher level of uncertainty in which it is desirable to know the entire range of possible costs, as well as the confidence limits on the estimate. This can best be done by application of probabilistic methods in the cost estimating process. The following discussion describes the probabilistic estimating method which TSB uses for complex projects and its advantages over the conventional deterministic cost estimating approach. An example of the use of these procedures in a deepwater well P&A cost estimate is presented.