This chapter provides an overview of the most popular indicators, such as the h-index and the journal impact factor, and how they have been developed. Over the last century, science has become increasingly important to society. Most countries allocate a substantial amount of their state budget to the production of new scientific knowledge and to the education of new generations of scientists. As public investment in science has increased, politicians, taxpayers, agencies and managers have become increasingly interested in how the funds allocated for research are spent, and what society gets in return. An important prerequisite for indicators that involve the number of citations is databases that track citations of academic publications. In creating such databases, the ideal of making an exhaustive database that includes all of the world’s academic journals and books needs to be balanced against what is feasible.