All authors want their submitted papers to have maximum impact. Perhaps the greatest complement you can earn as an author is to have your paper cited by other authors whom you respect. The impact of your paper can be on your professional reputation, your company’s ability to solicit customers and/or enlist partners, and if you are a university or government employee, on your yearly performance evaluation. For these reasons, authors sometimes consider journals’ impact factors when deciding to which journal they will submit. Institutions often use impact factors when evaluating professors for tenure. Librarians often consider them when making subscription decisions. Where did impact factors originate, and what do these numbers really mean?