Scientific knowledge is an important basis for our society. But not all scientific knowledge is equally important, and not all scientists are equally good scientists. In deciding which scientists to hire, which journals to acquire for a library, or which publications to read, various numerical indicators are often used to measure quality, impact, or relevance. This chapter provides a short overview of the most popular indicators, such as the h-index and the journal impact factor, and describe how they have been developed. On this basis, I discuss what the various indicators actually measure, where and to what extent it makes sense to draw on such numerical indicators, and where they may lead us astray.