Recent requirements of seismic risk estimation have led to a re-evaluation of historical earthquake records and statistical methods in many countries, with a view to optimizing the use of the available information. Whatever approach is chosen to quantify risk, the basic information is earthquake catalogs from which a recurrence relation is derived. Its most widely used form is still the Gutenberg-Richter loglinear relation, log N = a - bm, perhaps with some modification at larger magnitudes.