  With the vast majority of scientific papers now available online, this paper
describes how the Web is allowing physicists and information providers to
measure more accurately the impact of these papers and their authors. Provides
a historical background of citation analysis, impact factor, new citation data
sources (e.g., Google Scholar, Scopus, NASA's Astrophysics Data System Abstract
Service, MathSciNet, ScienceDirect, SciFinder Scholar, Scitation/SPIN, and
SPIRES-HEP), as well as h-index, g-index, and a-index.
