All living things on Earth are composed of cells. A cell is the simplest unit of a self-contained living organism, and the vast majority of life on Earth consists of single-celled microbes, mostly bacteria. These consist of a simple 'prokaryotic' cell, with no nucleus. The bodies of more complex plants and animals consist of billions of 'eukaryotic' cells, of varying kinds, adapted to fill different roles — red blood cells, muscle cells, branched neurons. Each cell is an astonishingly complex chemical factory, the activities of which we have only begun to unravel in the past fifty years or so through modern techniques of microscopy, biochemistry, and molecular biology. In this Very Short Introduction, Terrence Allen and Graham Cowling describe the nature of cells — their basic structure, their varying forms, their division, their differentiation from initially highly flexible stem cells, their signalling, and programmed death. Cells are the basic constituent of life, and understanding cells and how they work is central to all biology and medicine.