Quantum chemistry has emerged in the last two decades as a versatile discipline with its considerable advantages in chemical and material sciences. Concurrently, with the advent of Nanotechnology and the increase in fossil fuel combustion and exhaust emissions, the importance of studying toxic molecules from a sub-molecular view is becoming increasingly important. This book describes the theory behind quantum mechanics, the most used methods of calculation and presents quantum toxicological examinations of nanoparticles and molecular carcinogens that are frequently encountered in polluted natural, rural and urban environments. By the unique blend of quantum sciences and toxicology, the reader is trained in revealing and understanding the toxic properties of molecules from a quantum chemical point of view.