  Several observatories around the globe started regular full-disc imaging of
the solar atmosphere in the Ca II K line in the early decades of the 20th
century. These observations are continued today at a few sites with either old
spectroheliographs or modern telescopes equipped with narrow-band filters. The
Ca II K time series are unique in representing long-term variations of the
Sun's chromospheric magnetic field. However, meaningful results from their
analysis require accurate processing of the available data and robust merging
of the information stored in different archives. This paper provides an
overview of the historical and modern full-disc Ca II K observations, with
focus on their quality and the main results obtained from their analysis over
the last decade.
