A macrocycle is, as defined by IUPAC, a cyclic macromolecule or a macromolecular cyclic portion of a molecule . In the chemical literature, organic chemists may consider any molecule containing a ring of seven or more atoms to be a macrocyle. Co-ordination chemists generally define a macrocycle more narrowly as a cyclic molecule with three or more potential donor atoms that can co-ordinate to a metal centre. This book brings together the latest research results from around the world.