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Over the past two decades the echelle spectrograph NES of the 6-m telescopewas used to perform high resolution spectroscopy of far evolved stars spanninga wide range of initial masses. The studies cover a diversity of stars withhigh mass-loss rates during the preceding and current stages of evolution. Allthese stars have extended atmospheres and structured circumstellar envelopesproduced by strong stellar winds. We have studied luminous blue variables nearthe Eddington limit; hot supergiants with B[e] phenomenon in spectra, which arevery likely intermediate-mass binary systems soon after the fast mass exchangestage; a group of yellow hypergiants, as well as an extensive sample oflow-mass post-AGB supergiants. The diverse nature of the types of these starswhose common feature is the presence of a circumstellar envelope makes thespectroscopy of such objects a comprehensive task. Such studies consist of manyetapes, which include not only determining the peculiarities of theiratmospheric chemical composition and understanding the role of supergiants inthe enrichment of the interstellar medium with freshly synthesized elements,but also the determination of the evolution status of the objects considered,as well as search for and analysis of spectroscopic manifestations of kinematicprocesses in their extended and unstable atmospheres and gas-dust envelopes. Wespectroscopically monitored selected objects to study in detail the instabilityof the kinematic state of the atmospheres of the stars considered. This reviewreports briefly the most significant observational obtained within theframework of the programs in 1998--2021.

Results of selected stellar spectroscopy programs at the 6-m telescope of SAO RAS performed with the NES echelle spectrograph