This chapter deals with the diachronic side of reciprocity in English. Rather than being a fully fl edged historical investigation into the available historical data, I will make use of extant studies on the relevant topics and information provided in historical handbooks and other reference works, as well as data extracted from historical corpora and texts. In addition, some more theoretical issues will be brought up and dealt with in greater detail. In general, this chapter is meant to provide the historical background for the characterization of how the expression of reciprocity is organized in PDE. My guiding assumption is that synchronic facts can only be fully explained if they are seen in the light of their diachronic developments.