This chapter looks at the great variety of forms that cells can show. It considers how cells are made visible under the microscope. The chapter discusses how can exploit the similarities of living things to achieve a coherent understanding of all forms of life on Earth—from the tiniest bacterium to the mightiest oak. All living things are built from cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two. If cells are the fundamental unit of living matter, then nothing less than a cell can truly be called living. All cells contain DNA as a store of genetic information and use it to guide the synthesis of RNA molecules and proteins. The chapter also presents an overview of the key concepts discussed in this book.