PRIMA/PACMAN is scheduled for commissioning on Paranal in late 2008 as part of the VLTI. In this paper, we discuss the important aspects of its astrometric data-reduction software. For example, the top-level requirements, interfaces to existing ESO software, data types, data levels, and data flow among the recipes dictate the overall design of any software package. In addition, the complexity of the PACMAN instrument, the long-term nature of astrometric observations, and the need to improve algorithms as the understanding of the hardware improves, impose additional requirements on the astrometric data-reduction software.