Essential Science Indicators® provides Internet access to a unique and comprehensive compilation of essential science performance statistics and science trends. The chief indicators of output, or productivity, are journal article publication counts. For influence and impact measures, Essential Science Indicators employs both total citation counts and cites per paper scores. The former reveals gross influence while the latter shows weighted influence, also called impact. It is important to recognize that the data in Essential Science Indicators are limited to Thomson Scientific-indexed journal articles only. No books, book chapters, or articles published in journals not indexed by Thomson Scientific are taken into account here, either in terms of publication or citation counts.