The review is devoted to the results and method of a theoretical investigation of x-ray spectra of multiplycharged ions, with the aid of which the main parameters of astrophysical and laboratory nonstationary plasma are investigated. A distinguishing freature of such spectra is the presence of characteristic lines--satellites--producedin radiative decay of autoionization states. Calculation methods are described, with which to identify reliably the spectra $\Delta\lambda/\lambda=10^{-4}$ at $\lambda\approx1\hat{\mathrm{A}}$. The principal mechanisms of satellite formation--dielectronic recombination and direct excitation of $K$ shells, are considered. The development of modern theory of electron-ion collisions makes it possible to calculate the intensities of the spectral lines with accuracy not worse than several percent. A comparison of the calculated and observed relative intensities of the satellites makes it possible to determine the temperature and density of the electrons, multiplicity distribution of the ions in the active regions and in x-ray bursts on the sun, and also in the dense laboratory (laser, etc.) plasma. A number of solar and laboratory spectra are analyzed.