Path decompositions of graphs are an important ingredient of dynamic programming algorithms for solving efficiently many NP-hard problems. Therefore, computing the pathwidth and associated path decomposition of graphs has both a theoretical and practical interest. In this article, we design a branch-and-bound algorithm that computes the exact pathwidth of graphs and a corresponding path decomposition. Our main contribution consists of several nontrivial techniques to reduce the size of the input graph (preprocessing) and to cut the exploration space during the search phase of the algorithm. We evaluate experimentally our algorithm by comparing it to existing algorithms of the literature. It appears from the simulations that our algorithm offers a significant gain with respect to previous work. In particular, it is able to compute the exact pathwidth of any graph with less than 60 nodes in a reasonable running time (≤ 10min on a standard laptop). Moreover, our algorithm achieves good performance when used as a heuristic (i.e., when returning best result found within bounded time limit). Our algorithm is not restricted to undirected graphs since it actually computes the directed pathwidth that generalizes the notion of pathwidth to digraphs.