Description: This paper presents an unbiased, consistent and asymptotically efficient estimator based on importance sampling variance reduction technique in the framework of Monte Carlo simulation methods. It shows that the estimator is highly efficient in terms of number of computations. Using the Pollaczeck-Khinchine formula, the authors extend the use of the estimator to the calculation of ultimate ruin probabilities in the context of the Classical case of Risk Theory and compare their results with recent actuarial literature using the same methodology.