This chapter deals with understanding the origins of chemical bonding in molecules and solids. Many trends in the properties of solids follow directly from trends in the properties of the constituent atoms. The chapter illustrates the square well as a prototype atom, and investigates the evolution of the energy level spectrum as the separation between two square wells is decreased to give a diatomic, or double square well potential. It considers bonding in a diatomic molecule made up of two different atoms, such as LiH. The chapter discusses how trends in the electronic properties of tetrahedrally bonded semiconductors can be understood in terms of trends in the covalent and ionic contributions to their bonding. It presents the hybrid orbital picture that provides a convincing explanation for the crystal structure of tetrahedrally bonded semiconductors, and provides insight into trends in a variety of semiconductor properties.