This chapter provides a conceptual and empirical overview of soils, soil fertility, and soil macro-fauna within cocoa-based agroforestry systems in Cameroon. It frames these components as critical to the ecological functionality and sustainability of cocoa production landscapes, particularly in tropical regions where soils are naturally nutrient-poor and prone to degradation. The chapter explores the dynamic interactions between soil properties, biological organisms, and agroforestry management practices, highlighting how these interactions determine long-term productivity and environmental resilience. The chapter begins by defining key concepts such as soil fertility, macro-fauna, and agroecosystem sustainability. It explains the importance of soil macro-fauna (including earthworms, termites, ants, and beetles), as ecological engineers that facilitate organic matter decomposition, soil aeration, nutrient cycling, and structural development. Their presence and diversity are emphasized as reliable indicators of soil health and management intensity. Empirical studies reviewed in the chapter show that cocoa agroforests with diverse tree cover and minimal chemical inputs tend to support higher macro-faunal abundance and diversity, which correlates with improved soil properties such as higher organic carbon, enhanced cation exchange capacity (CEC), and better moisture retention. In contrast, conventional monocultures with intensive agrochemical use exhibit reduced biodiversity, soil acidification, and nutrient imbalances. The chapter also underscores how macro-fauna mediate key processes that improve cocoa yields and system resilience, emphasizing their role in agroecological intensification. However, it notes significant gaps in quantitative research linking specific soil fauna groups to soil fertility outcomes, particularly in the Cameroonian context. This chapter builds a scientific foundation for understanding how biodiversity belowground influences sustainability aboveground and argues for integrated management approaches that enhance soil life as a pathway to regenerative cocoa production.