In the first half of this century, great strides were made in under­ standing the behavior of polymers in dilute solutions or in the solid state. Concentrated solutions, on the other hand, were commonly regarded as mainly of interest to practitioners, being too complex for the rigorous application of statistical theory. Given the preoccupation with the isolated polymer molecule and the attendant focus on the state of infinite dilution, it is not surprising that aggregation, and inter-polymer associ­ ation in general, was the bugaboo of experimentalists. These attitudes have changed remarkably over the last few decades. The application of sealing theory to polymer solutions has stimulated investigation of the semi-dilute state, and the region between infinite dilution and swollen gel is no longer perceived as terra incognita. New techniques, such as dynamic light scattering, have proven to be of much value in such investigations. At the same time, it has become clear that consideration of strong inter- and intra-polymer forces, superimposed on the familiar description of the statistical chain, is prerequisite to the application of polymer science to numerous systems of interest. Para­ mount among these, of course, are biopolymers, their complexes and assemblies. The isolated random coil must be viewed as tl rarity in nature.