Common practice in tunnel design considers a deterministic approach for dimensioning primary support and final lining. Generally the design is based on some parameters statistics (for example the mean) and a fixed factor of safety is required to take into account different sources of uncertainty. However, this index is not sufficient to quantify the reliability of a support structure, and can be easily shown that to the same factor of safety could correspond different values of probability of failure. In this paper, the basic concepts of the probabilistic design are shortly reviewed and some practical applications in tunneling are described as developed by the authors, showing the advantages obtained with respect to implementation of the traditional deterministic approach to the same cases.