The dependency of human health on basic ecosystem services is the most fundamental, most catastrophic if threatened, and often most overlooked function of green infrastructure (GI) presented in this book. Ecological models of health reflect the fundamental importance of ecosystems to health and the role of the natural environment in supporting them, but it has only been within the past decade that public health has begun to seriously confront the threat to health posed by degraded ecosystems. GI plays a vital role in the continued provision and control of the quantity and quality of the most essential of life-supporting elements: water. The role GI plays in regulating water quantity stems foremost from its role in the hydrological cycle, but GI is important to water quantity because it facilitates the recharge of groundwater stores and aids in controlling surface runoff volumes. GI supports water quality through its ability to filter pollutants from rainfall and pollutants that are collected in surface runoff.