Results are presented of radio tomographic (RT) studies of the ionosphere during quiet and disturbed time intervals. The existing navigational systems and ground-based receiving networks make it possible to obtain a series of tomographic data and to reconstruct the distributions of refractive index of the near-Earth plasma. Methods for ionospheric radio tomography (RT) using low-orbital satellite systems (LORT) such as the Russian "Tsikada" or American "Transit", and radio tomography based on high-orbital satellite systems (HORT) like GPS/GLONASS are considered. Numerous LORT experiments afforded an opportunity to study both the known ionospheric structures (troughs, crests of equatorial anomaly, travelling ionospheric disturbances, patches, blobes, traces of precipitation and so on) and to investigate a series of little known ones such as wavelike structures, tilted ionization troughs and others. Strong disturbances of ionospheric plasma caused by anthropogenic factors, in particular, effects of rocket launchings, industrial explosions, powerful HF radiation and so on, are studied. LORT also provides a possibility to determine plasma fluxes. Development of the global navigation systems GPS, GLONASS and Galileo places at researcher.s disposal new investigation instrument. Low angular velocity of GPS satellites motion makes it essential to allow for temporal variations in the ionosphere, which necessarily leads to the statement of 4D tomography problem (three spatial coordinates and time). However, unlike in 2D LORT, here an additional procedure is needed to interpolate the found solutions into regions of missing data. In the talk, examples of 4D reconstruction of the ionosphere from GPS data are given, and HORT results are compared with the data of other measurements. Examples of LORT results comparison with HORT are shown. It is shown that combination of LORT and HORT provides best quality ionospheric imaging. LORT and HORT results obtained in different latidudinal and longitudinal sectors are compared with corresponding model data (IRI, Nequick, IFM, GAIM, GIM). Scenarios for RT with elements of radio occultation technique are described. Problem of RT data assimilation is discussed.