During the past decade or so, many measurements of Galactic HI absorption
using VLBI against extra-galactic sources as well as those using
multi-epoch observations in pulsar directions have reported detectable
differences in the absorption when two or more sight-lines with small
angular separations are compared. The angular separations translate to
transverse spatial scales down to tens of AU at the distance of the
absorbing matter. These measurements have invariably been interpreted as
evidence for significant wide-spread small-scale structure in the HI
distribution with volume densities orders of magnitude higher ($n_{HI}\sim
10^4-10^5$ cm$^{-3}$) than those implied by the parsec-scale component.
Naturally, there are serious difficulties about such structures being in
equilibrium with the other components of the medium, and hence about how
to generate and maintain such apparently commonly encountered structures.
Some recent suggestions for a geometric solution (invoking cold, dense
curved filaments or sheets that line-up along sight-lines), while easing
some, appear to have their own set of difficulties.
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In this talk, we will see how the observations have been {\it
mis-interpreted},
particularly in estimating and attributing such high densities to the
structure on the sub-parsec scale, making it appear as a physically
distinct population of structures. We will also show that the observed
opacity differences, {\it when interpreted correctly}, are not surprising,
but are consistent with a single power-law description of the distribution
of HI-opacity in the interstellar medium. There appears to be no
compelling
evidence to treat the HI structure at small-scales (tens to hundreds
of AU) as a component distinct from the rest of the neutral medium. In
fact, the small-scales contribute only a tiny fraction of the observed
column densities along any sight-line, consistent with  their equilibrium
coexistence with the wide range of hierarchical scales.
This, in our opinion, should {\it put an end to the decades long puzzle of
the so-called small-scale structure} in HI and other species in the
Galaxy.