A s you read this article, millions of your cells are dying. Relax. Most are sacrificing themselves to ensure your survival. Burgeoning research indicates that the health of all multicellular organisms, including humans, depends not only on the body's ability to produce new cells but on the ability of individual cells to self-destruct when they become superfluous or disordered. This critical process, today called apoptosis, or programmed cell death, was overlooked for decades. But biologists have recently made rapid strides in understanding how cellular suicide is enacted and controlled. Many investigators are motivated both by scientific curiosity and by a desire to combat some of the world's most frightening diseases. It turns out that aberrant regulation of apoptosis—lead-ing to too much or too little cell sui-cide—probably contributes to such varied disorders as cancer, AIDS, Alzheim-er's disease and rheumatoid arthritis. Researchers who studied embryonic development in the first half of the 20th century were the earliest to realize that cell death is not, as had long been assumed , invariably bad for the body; in fact, it is necessary. By the 1950s, they had shown that multicellular creatures obtain their final form by predictably eliminating selected cells. The tadpole deletes its tail during transformation into a frog; mammals erase countless neu-rons as the nervous system takes shape. Microscopists had also identified major signposts distinguishing this physiological cell death from accidental destruction , or necrosis. Necrotic death occurs when a cell is severely injured, by a physical blow or by oxygen deprivation, for example. Swelling is a defining feature. Internal organelles—most obviously the mito-chondria (the cell's power plants)—and the entire cell balloon and rupture. These effects occur because injury prevents the cell from controlling its fluid and ion balance; water and charged particles (especially sodium and calcium ions) that are normally pumped out now stream in. Another hallmark is inflammation: circulating macrophages and other white blood cells of the immune system converge on the necrotic cells and ingest them. Inflammation helps to limit infection and clear away debris, but the activities and secretions of the white cells can also damage normal tissue in the vicinity, sometimes extensively. Scientists viewing the cell undergoing apoptosis see very different changes. They find no swelling. Instead the dying cell shrinks and pulls away from its neighbors. Soon it appears to boil: blebs form on the surface and disappear, immediately replaced by others. Internal organelles retain their …