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Savannas are characterized by the coexistence of trees and grasses and occur largely in the seasonal tropics between the equatorial rainforests and mid-latitude desert ecosystems. This article describes the defining elements of the savanna biome and documents global savanna distribution and relationships with climate. It examines the influence of the four dominant environmental factors that determine savanna structure and function; available moisture and nutrients, fire occurrence and impacts from herbivory, and the interactions between these environmental determinants. Savannas can be considered as a metastable mixture of trees and grasses, and theories advanced to explain the stability, or otherwise, of this coexistence are discussed. Current issues in savanna management and threats to long-term sustainability are also described.