A general mathematical model was used to study the spectral efficiency of cellular systems. The model took into consideration effects of first and second tier interference. The random distribution of users across the whole area covered by the cellular system as well as the shadowing and multipath effects were taken into account. The influence of sectorization on value of the spectral efficiency was also studied. The model investigated the effects of the cell radius and cluster size on the spectral efficiency. The influence of different propagation conditions on the spectral efficiency was also considered. Results of simulation showed that an improvement of up to 70% can be achieved in the spectral efficiency, when using a six-sector system in comparison with the omni-directional system. Also, the spectral efficiency was shown to decay by a rate proportional to the square of the radius. It was also shown that spectral efficiency improved in severe propagation conditions. Using a higher value for the cluster size decreased the spectral efficiency by a rate proportional to the square root of the cluster size.