Soils form a relatively thin layer that acts as the interface between the atmosphere and lithosphere. They provide a wide range of ecosystem goods and services that both support human societies and underpin the terrestrial components of the earth system. Soils are important to local, national and global security since they protect food and fuel supply, underwrite environmental quality and enhance culture. Furthermore soils have security-related roles in environmental and criminal forensics and as a potential receiver compartment of hostile biological or chemical agents. Soils are remarkably complex in terms of their biological, chemical and physical constitution. Biodiversity belowground always exceeds that found above the surface, particularly at the microbial scale. Soil organic matter is extremely varied, being comprised of the simplest organic molecules through to vast randomly arranged structured polymers that can be highly resistant to decomposition. The physical structure of soils is manifest as pore networks of highly complex geometry that are connected across spatial scales that typically span several orders-of-magnitude. Soils exhibit properties characteristic of complex systems including indeterminacy, non-linearity, emergent behaviour and self-organization. They can serve as a potent model to study complexity, with the added incentive that there is an imperative to manage them sustainably.