  Our knowledge of the long-term evolution of solar activity and of its primary
modulation, the 11-year cycle, largely depends on a single direct observational
record: the visual sunspot counts that retrace the last 4 centuries, since the
invention of the astronomical telescope. Currently, this activity index is
available in two main forms: the International Sunspot Number initiated by R.
Wolf in 1849 and the Group Number constructed more recently by Hoyt and
Schatten (1998a,b). Unfortunately, those two series do not match by various
aspects, inducing confusions and contradictions when used in crucial
contemporary studies of the solar dynamo or of the solar forcing on the Earth
climate. Recently, new efforts have been undertaken to diagnose and correct
flaws and biases affecting both sunspot series, in the framework of a series of
dedicated Sunspot Number Workshops. Here, we present a global overview of our
current understanding of the sunspot number calibration. While the early part
of the sunspot record before 1800 is still characterized by large uncertainties
due to poorly observed periods, the more recent sunspot numbers are mainly
affected by three main inhomogeneities: in 1880-1915 for the Group Number and
in 1947 and 1980-2014 for the Sunspot Number. The newly corrected series
clearly indicates a progressive decline of solar activity before the onset of
the Maunder Minimum, while the slowly rising trend of the activity after the
Maunder Minimum is strongly reduced, suggesting that by the mid 18th century,
solar activity had already returned to the level of those observed in recent
solar cycles in the 20th century. We finally conclude with future prospects
opened by this epochal revision of the Sunspot Number, the first one since Wolf
himself, and its reconciliation with the Group Number, a long-awaited
modernization that will feed solar cycle research into the 21st century.
