  In this presentation we review the link between the statistics of intensity
fluctuations in spectral line data cubes with underlying statistical properties
of turbulence in the interstellar medium. Both the formalism of Velocity
Channel Analysis for optically thin lines and its extension to the lines with
self-absorption is described. We demonstrate that by observing optically thin
lines from cold gas in sufficiently narrow (thin) velocity channels one may
recover the scaling of the stochastic velocities from turbulent cascade, in
particular, Kolmogorov velocities give $K^{-2.7}$ contribution to the intensity
power spectrum. Synthetically increasing the channel thickness separates out
the underlying density inhomogeneities of the gas. Effects of self absorption,
on the other hand, retain the velocity signature even for integrated lines. As
a result, intensity fluctuations tend to show universal but featureless scaling
of the power $\propto K^{-3}$ over the range of scales.
