Recent years there appeared many experimental and theoretical studies of highly excited vibrational states of small polyatomic molecules (Refs. (1 -4) and references therein). These studies have been stimulated by current interest in intramolecular kinetics, photoselective chemistry, and atmospheric optics. The investigation of highly excited vibrationrotational states has become important for searching a quantum analog of classic chaotic motion, classic-quantum correspondence, nonlinear resonance studies and for various application in the laser chemistry, environment, atmospheric optics, and astrophysics. Rotational-vibrational lines caused by transitions to excited vibrational states (energy is larger than 7000 cm') are very weak, their strengths, as a rule, are 5 -7 order of magnitude less than line strengths of fundamental bands. Therefore, high sensitive spectrometers must be used to record these weak bands. Intracavity laser spectrometers due to their high sensitivity to absorption become one of the powerful tools for studying highly excited molecular states. The method of broad-band Intracavity Laser (ICL) Spectroscopy consists of quenching the laser emission at the absorption-line frequencies of the species placed in a broad-band laser cavity. In this case the laser emission spectrum has sharp dips at the frequencies of absorption lines, which can be recorded with an ordinary spectroscopic instrumentation. In the intracavity laser spectroscopy a laser itself is a nonlinear detector of weak absorption. The laser imitates a multipass absorption cell, but with much greater effective length Leff Of the absorbing layer than in a simple multipass cell where it is limited by radiation losses on mirrors. In this case Leff S proportional to the duration t of continuous generation in the region of an absorption line under study. ICL -spectroscopy was widely used to various applications some of them are described in the monograph5 and in other papers of the present issue. One of the most effective applications of ICL - spectroscopy is study of weak molecular spectra caused by transitions to highly excited states. This paper summarized investigation of highly excited molecular states using the intracavity laser spectroscopy.