Bacteria form a fundamental branch of life. They are the oldest forms of life as we know it, and they are still the most prolific living organisms. They inhabit every part of the Earth's surface, its ocean depths, and even terrains such as boiling hot springs. They are most familiar as agents of disease, but benign bacteria are critical to the recycling of elements and all ecology, as well as to human health. In this Very Short Introduction, Sebastian Amyes explores the nature of bacteria, their origin and evolution, bacteria in the environment, and bacteria and disease. In looking at our efforts to manage co-evolving bacteria, he also considers the challenges of resistance to antibiotics.