This chapter discusses how performance is measured in science, such as through the role of citation metrics. Next, the chapter discusses the pros and cons of bibliometric indexes, and of ‘impact factor’, which was introduced by Eugene Garfield in 1955 but not widely used until twenty years later. The various ways that journals attempt to improve their impact factors, and how this will affect science, are also examined. Besides impact factor, the role played by indicators in evaluating scientists, such as the recently introduced h-index, is explored. Finally, fashions and trends in science are touched upon, illustrated with personal anecdotes from the author.