In current catalogs, the parameters of preinstrumental earthquakes represent the final synthesis of earthquake records of very different type, quality, and reliability. Parameters may be derived from actual and contemporary observations of earthquake effects interpreted as intensity distributions or may be supplied by later seismological studies and descriptive catalogs, consisting of the reinterpretation of already interpreted accounts. The content of earthquake catalogs results from piling data of different origin, interpreted according to diverse approaches. Consequently, the homogeneity and reliability of earthquake parameters strongly vary from one region or period to another, and it is not as straightforward as assumed by end users. The analysis of the wealth of data in the European Archive of Historical Earthquake Data (AHEAD) evidences many differences in their typology and update of the studies supporting European preinstrumental earthquake catalogs, for both moderate and large earthquakes, over a period of 1000 yr.