  The final analysis of the Extensive Air Shower (EAS) maximum X_max depth
distribution derived from the data of Tunka-25 atmospheric Cherenkov light
array in the energy range 3.10^15 - 3.10^16 eV is presented. The perspectives
of X_max studies with the new Cherenkov light array Tunka-133 of 1 km^2 area,
extending the measurements up to 10^18 eV, are discussed.
