This book focuses on the theoretical problems associated with molecular diversity as it is being applied in the pharmaceutical industry. Therefore, this book deals with algorithms that are involved in understanding chemical space and selection of diverse sets of structures. The algorithms also deal with the problem of focused diversity where chemical libraries are being created within a structured physical volume. Diversity is necessarily connected to combinational chemistry, although this book is limited to the application of diversity methods to combinational chemistry and does not deal with synthetic methods. It is this focus on algorithms and strategies for exploiting molecular diversity that makes it different from books on combinational chemistry. The intended readership of the book falls into two categories: those actively engaged in applying molecular diversity in the chemical industry and those in academia who are developing strategies to embrace, understand and accept the many problems thrown up by this new research field of molecular diversity.