In the living world, the mechanisms that make us breathe, digest, grow or move an arm involve molecules made up of thousands of atoms, called macromolecules. These macromolecules are constantly circulating, interacting and deforming. Visualising these macromolecules at the atomic level is a unique tool for understanding their function and for designing drugs, inhibitors or activators. When using the three-dimensional structure of a macromolecule, it is essential to keep a critical eye. To do this, we need to understand how the structure was obtained. X-ray crystallography is a historical method of using X-rays and crystals to determine the three-dimensional atomic structure of molecules. It is also the most widely used.The main audience for this book is biological scientists, although it will appeal to anyone interested in structural biology. It introduces the different steps involved in biological crystallography, from crystallisation to the determination of the three-dimensional structure of a macromolecule. Thanks to the videos available through the QR codes at the end of each chapter, sites normally closed to the public are opened up to offer you a unique journey to the heart of life.