In the vast majority of airports where demand exceeds capacity, landing and take-off slots are allocated ac- cording to the process established by the Worldwide Slot Guidelines (WSG). An optimization-based decision- support tool is introduced in this paper to assist slot coordinators in the initial slot allocation – the most important stage of that process. The WSG comprises both primary and additional criteria to be taken into account by slot coordinators at this stage. Considering all additional criteria specified in the WSG, this tool generates, in an acceptable amount of time, alternative efficient slot allocations against which the coordinators can evaluate the solutions obtained through the heuristic methods they apply. The decision-support tool also offers solution analysis features, such as visualization, ranking, comparison of multiple optimal solutions, and comparison of solutions regarding important characteristics not reflected in the WSG additional criteria. The tool is being tested at one of the largest airports worldwide – Guarulhos (Sao Paulo, Brazil) – with results that clearly illustrate its usefulness in practice.