Hundreds of different types of cell, each based on a single design and all derived from one fertilised egg, make up our bodies. Their sheer variety, exquisite minuteness, versatility, and fortitude make cells truly extraordinary. Our entire existence depends on the actions of cells with precisely defined roles that are performed at very particular places in our bodies, governed by a vast network of chemical messages. Ultimately, a cellular clock determines how many times they can divide, and a program exists that can sacrifice cells for the benefit of the entire bodily organisation. When we are sick, it is really our cells that are sick, and it is in cells that scientists look for the immediate cause of most diseases. This chapter is concerned with the strengths and weaknesses of cells in health and disease.