Symbiosis, the sustained and intimate associations between unrelated life forms, is now recognized as a ubiquitous phenomenon, one that has shaped evolution since the origin of life and that continues to affect all species. This overview of symbiosis starts with a quick history of relevant early discoveries and researchers, and considers why symbiosis was so long neglected as a respectable topic of biological research and why it was a controversial topic. Today, symbiosis is widely appreciated as being everywhere in nature and as a pervasive influence on ecological communities. One chapter explores the fundamental drivers that lead to symbiotic associations, using examples to illustrate the nature of services exchanged between symbiotic partners. Another considers the evolutionary stability of symbiotic partnerships, which can quickly decay in the face of symbiotic cheating. A full chapter is devoted to the most consequential of all symbioses: the origin of the complex (eukaryotic) cell, and the origin of chloroplasts and green plants. Other triumphs of symbiosis described include the root-fungus associations that enabled plants to colonize land 450 million years ago, gut microbial communities that empower animals to utilize a wide range of foods, including plant fiber wood and sap, and coral-algal symbioses that resulted in the rise of coral reefs.