The book gives an original interpretation of modern ideas about oligomers as substances which, due to their molecular structure, exhibit peculiarities of statistical behaviour different form the statistics of monomers and polymers. The criteria are given that enable one to distinguish between the ranges of monomers, oligomers and polymers in homologous series. Theoretical aspects of a small-scale oligomer structure of and models for the formation of the supermolecular and the organization of liquid oligomeric systems are presented. The theoretical and experimental results of studying phase transitions and anomalies of physicochemical properties inherent to oligomeric systems are considered.