This book explores the geography, ecology, and antiquity of ‘open ecosystems’, which include grasslands, savannas, and shrublands. They occur in climates that can support closed forest ecosystems and often form mosaics with forest patches. With the aid of remote sensing, it is now clear that open ecosystems are a global phenomenon and occur over vast areas in climates that could also support forests. This book goes beyond regional narratives and seeks general explanations for their existence. It develops the theme of open ecosystems as being widespread and ancient, with a distinct biota from that of closed forests. It examines hypotheses for their maintenance in climate zones favouring the development of forests, including soils hostile for tree growth, fire, and vertebrate herbivory.