AbstractIn the school timetabling problem a set of lessons (combinations of classes, teachers, subjects and rooms) has to be scheduled within the school week. Considering classes, teachers and rooms as resources for the lessons, the problem may be viewed as the scheduling of a project subject to resource constraints. We have developed an algorithm with three phases. In Phase I an initial solution is built by using the scheme of parallel heuristic algorithm with priority rules, but imbedding at each period the construction of a maximum cardinality independent set on a resource graph. In Phase II a tabu search procedure starts from the solution of Phase I and obtains a feasible solution to the problem. The solution obtained is improved in Phase III. Several procedures based on the calculation of negative cost cycles and shortest paths in a solution graph are used to get more compact timetables.The algorithms have been imbedded in a package designed to solve the problem for Spanish secondary schools. The computational results show its performance on a set of real problems. Nevertheless, it can be applied to more general problems and results on a set of large random problems are also provided.