The properties of the macroscopic materials are the features of interest to the engineer. Materials are made up of atoms and the nature of these atoms and the way that they are bonded together to make a macroscopic material dictates the properties of the material. To make materials useful to engineers, they need to be shaped into components. Since the nature of the bonding defines the physical, mechanical properties, there is similarity in some of the properties for materials in the same class. It should be noted that while some of the characteristic properties of materials in a particular class might be broadly the same, there can also be a wide variation in basic properties. Polymer chains often arrange themselves in tangles with no regularity, particularly if the bonding between the chains is strong. Polymers with weak van der Waals bonding between the chains, polyethylene for example, are capable of forming ordered structures if cooled slowly from the liquid state. © 2023 selection and editorial matter, Michael Clifford; individual chapters, the contributors.