  After more than half a century of community support related to the science of
"solar activity'', IAU's Commission 10 was formally discontinued in 2015, to be
succeeded by C.E2 with the same area of responsibility. On this occasion, we
look back at the growth of the scientific disciplines involved around the world
over almost a full century. Solar activity and fields of research looking into
the related physics of the heliosphere continue to be vibrant and growing, with
currently over 2,000 refereed publications appearing per year from over 4,000
unique authors, publishing in dozens of distinct journals and meeting in dozens
of workshops and conferences each year. The size of the rapidly growing
community and of the observational and computational data volumes, along with
the multitude of connections into other branches of astrophysics, pose
significant challenges; aspects of these challenges are beginning to be
addressed through, among others, the development of new systems of literature
reviews, machine-searchable archives for data and publications, and virtual
observatories. As customary in these reports, we highlight some of the research
topics that have seen particular interest over the most recent triennium,
specifically active-region magnetic fields, coronal thermal structure, coronal
seismology, flares and eruptions, and the variability of solar activity on long
time scales. We close with a collection of developments, discoveries, and
surprises that illustrate the range and dynamics of the discipline.

Solar and Stellar Astrophysics