History is commonly described in many scientific texts. Yet it is often not clear why. In this chapter, six reasons for studying history of a science are distinguished. On the one hand, there are three kinds of history use which reflect identity building on the part of the author rather than any contribution to the content of scientific knowledge. These uses of history are either neutral or hinder the progress of science. On the other hand, three kinds of studies of the history of science are distinguished that are essential for the development of scientific knowledge. In the last part of the chapter, the idea of scientific progress is discussed, and it is shown that only historical science can be progressive. Ahistorical science, which usually uses history only for identity building, becomes noncumulative and fragmented and increasingly ceases to be science.