All polymer molecules have unique features of one sort or another at the level of the individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter, the author shall focus attention on two other situations that introduce structural variation at the level of the repeat unit: the presence of two different monomers, and the regulation of configuration of successive repeat units. Copolymerization offers a facile means to tune material properties, as the average composition of the resulting polymers can often be varied across the complete composition range. Similarly, control of stereoregularity plays an essential role in dictating the crystallinity of the resulting material, which in turn can exert a profound influence on the resulting physical properties. For both copolymers and stereoregular polymers, experimental methods for characterizing the products often involve spectroscopy.