  First such observations were made in 1892 and since then various sites around
the world have carried out regular observations, with Kodaikanal, Meudon, Mt
Wilson, and Coimbra being some of the most prominent ones. By now, Ca II K
observations from over 40 different sites allow an almost complete daily
coverage of the last century. Ca II K images provide direct information on
plage and network regions on the Sun and, through their connection to solar
surface magnetic field, offer an excellent opportunity to study solar magnetism
over more than a century. This makes them also extremely important, among
others, for solar irradiance reconstructions and studies of the solar influence
on Earth's climate. However, these data also suffer from numerous issues, which
for a long time have hampered their analysis. Without properly addressing these
issues, Ca II K data cannot be used to their full potential. Here, we first
provide an overview of the currently known Ca II K data archives and sources of
the inhomogeneities in the data, before discussing existing processing
techniques, followed by a recap of the main results derived with such data so
far.
