  The ring-diagram technique was developed by Frank Hill 25 years ago and
developed quickly during the late 1990s. It is nowadays one of the most
commonly used techniques in local helioseismology. The method consists in the
power spectral analysis of solar acoustic oscillations on small regions (2 to
30 degrees) of the solar surface. The power spectrum resembles a set of
trumpets nested inside each other and, for a given frequency, it looks like a
ring, hence the technique's name. It provides information on the horizontal
flow field and thermodynamic structure in the layers immediately below the
photosphere. With data regularly provided by MDI (on board SOHO), GONG+ network
and more recently HMI (on SDO), many important results have been achieved. In
recently years, these results include estimations of the meridional circulation
and its evolution with solar cycle; flows associated with active regions, as
well as, flow divergence and vorticity; and thermal structure beneath and
around active regions. Much progress is expected with data now provided by
HMI's high spatial resolution observations and high duty cycle. There are two
data processing pipelines (GONG and HMI) providing free access to the data and
the results of the ring-diagram analysis. Here we will discuss the most recent
results and improvements in the technique, as well as, the many challenges that
still remain.
