Abstract
Certain current questions concerning the theory of highly excited vibrational states have been examined. The change in the form and the objectives of the theory as a function of increased energy of the system has been analysed. Much attention has focused on the role of classical concepts in regard to both the definition of the objectives and the formation of concepts and models used to interpret experimental data in the field of high-energy states. The concept of localised quantum states has been introduced. This concept plays a key role in the interpretation of many processes in which highly excited vibrational states are involved. The experimental consequences of the formation of localised quantum states in different physical situations are discussed. Examples examined include rearrangements of vibrational polyads, the appearance of periodic orbits in wave functions and in low resolution spectra, and bound states in continua. The bibliography includes 208 references.