The purpose of my dissertation is to improve understanding of historical, current, and future infill and sprawl expansion and their causes across primary and secondary cities. The West Africa urban system (WAUS) is a global hotspot of rapid urbanization and urban expansion. Although urbanization is associated with industrialization and economic growth, most African governments have limited resources, services, and infrastructure to manage the negative impacts (e.g., biodiversity, habitat, and cropland losses) associated with urban expansion, including sprawl that expands the urban footprint and infills within previous urban developments. Previous studies on urban expansion in West Africa have mostly focused on a few individual primary cities with over one million urban populations. The numerous secondary cities with less than one million urban population are also important components of the urban system, providing services, markets, and education centers for many rural residents in West Africa. Also, cities are increasingly interconnected, and changes and associated impacts extend beyond localized scales, requiring detailed understanding of urban expansion dynamics for the entire network of cities. This research achieved the purpose of the dissertation in three interconnected ways. First, I used satellite data to comprehensively analyze infill and sprawl expansion from 2001 - 2020 across 1603 cities in Ghana, Togo, Benin, and Nigeria. The findings from this research show that more than half (54%) of the expanded area occurred in smaller cities, and 73% was sprawl. Sprawl-to-infill ratios were higher in smaller cities than in larger cities, and the annual expansion rates of larger cities decreased over time while those in smaller cities were stable or increased. This research also found that proximity to larger cities increased smaller cities' expansion rates in Nigeria, but more remote cities had higher expansion rates in Ghana, Benin, and Togo. Second, I used a mixed-method approach to understand the causes of sprawl and infill expansion in primary and secondary cities, using Ghana as a case study. In primary cities, high cost of land and rent, travel costs, and fewer land allocation regimes were important considerations, whereas in secondary cities, lower land, rent, and travel costs and more diverse land allocation were important factors. These factors are more favorable in the periphery because land and rental costs are mostly cheaper, especially in secondary cities; this likely explains the higher sprawl-to-infill expansion. Interviews also suggest that new developments reflected the aims of the powerful urban stakeholders, including the chiefs and political elite. Third, this research used FUTure Urban-Regional and Environment Simulation (FUTURES) to model future urban expansion across Ghana, Togo, Benin, and Nigeria from 2020 – 2050. This research finds that input variables varied across small, medium, and large cities. New developments were mostly influenced by proximity to previous developments, whereas elevation and hillshade were generally less important across cities. This research also finds high annual expansion rates of 3.8% to 14.3% across the study area, with most expansion occurring between 2030 – 2040. Overall, this dissertation research provides a detailed analysis of the historical, current, and future urban expansion of cities with varying population sizes in WAUS. It finds that smaller and more numerous cities contribute substantially to urban expansion and need to be incorporated into national and regional assessments of urban growth and its impacts.