This report, part of the "Cities" collection, provides an analysis of the demographic and morphological changes in West African border cities since the mid-20th century. Using the Africapolis harmonised database makes it possible to show that since 1950 border cities have experienced higher rates of growth than other cities in the region. While the average size of cities increases with distance from a border, the opposite is true for urban density; it decreases as distance from a border increases. This suggests that border cities form urban centres that differ from other such centres due to the fact that they specialise in the commercial activities that stimulate growth and foster higher densities. The report identifies the 27 main cross-border agglomerations in the region and discusses their specific characteristics. Also in this Collection: "Regional Integration in Border Cities", No. 20 "Businesses and Health in Border Cities", No. 22 "Accessibility and Infrastructure in Border Cities", No. 23