In the past usually the queueing theory was used to determine the number of berths and with that the quay length. This analytic method can be applied when it is acceptable to create a very simple model by schematization of the so called real life system. In most cases however it is not possible to schematize to such an extend that the model consists only of a queue (anchorage) and a number of places where service is rendered to ships (berths). Moreover, it is necessary to express the inter-arrival time distribution and the service time distribution in mathematical formulations. A powerful method that can be used in complicated situations is offered by the discrete computer simulation of harbour systems, provided that enough data are available. In this paper a review is given of the situations that can be solved in an analytical way by means of the queueing theory. Next the discrete computer simulation technique will be discussed in the course of which the possibilities and limitations of this technique when applied to harbours in developing countries come up. The simulation results of the harbour of Beira in Mozambique are discussed to illustrate the possibilities. A generally applicable simulation model, developed by the Delft University of Technology has been used to execute the simulation.