What are cells made of? Biologists have been studying cells since the mid-1600s, when Robert Hooke viewed a slice of cork through a microscope and coined the word "cell" to describe the walled-in spaces he saw. Most cells are invisible to the naked eye. Yet they carry out the many complex processes that make life possible. As microscopes have improved, scientists have learned more and more about cells and their organelles—the structures within cells. From the nucleus, the cell's control center, to the tiny ribosomes, which help manufacture proteins, each part of a cell plays an essential role. This book tells the story of how biologists unlocked the secrets of cells and revolutionized the way we look at living things.