This book deals with a truly interdisciplinary subject: protein condensation from solution. We use “condensation” in this book to denote one of several forms of proteins: a dense, protein-rich fluid phase, an amorphous aggregate, a gel, a crystal, or a polymer fiber. All these forms have been observed experimentally and are important in their own right. The primary purpose of the book is to bring to a wide audience the current status of research in the field, which is still evolving at a rapid rate. The bulk of the book deals with issues related to producing high quality protein crystals from solution, in which the bottleneck is crystal nucleation. Here the main challenge is to determine the initial solution conditions so that optimal crystal nucleation occurs. A second and increasingly important subject that we discuss involves diseases that occur due to undesired protein nucleation. A classic example is the nucleation of polymer fibers of sickle hemoglobin molecules within the red blood cells that distorts the cells and produces sickle cell anemia. Another example is that of age-related cataracts produced by the undesired aggregation of γ-crystallin protein molecules within the vitreous fluid of the eye. A third, somewhat different, example involves the role of amyloid β protein in Alzheimer's disease.