Recent changes in farming throughout Europe are threatening steppe birds, such as Montagu's Harrier Circus pygargus, a steppe raptor that nests in cereal crops, and is therefore threatened by the destruction of its nests by harvesters. Here we analyse the reproductive biology of Montagu's Harrier in Granada province of (south-eastern Spain), based on 184 nests monitored from 2003 to 2014. The mean laying date in our study area is one of the earliest recorded for the species, and mean clutch size one of the lowest, as expected considering the latitude at which the population is located. We also found that laying date was progressively earlier over the study period, whereas both clutch size and productivity have increased. The results show high reproductive success (82%), with a productivity of 2.67 fledglings per nest. However, this high productivity was achieved thanks to the nest rescue campaigns. Had the nests saved been lost, productivity would be only 1.47 fledglings per nest. Therefore, nest-rescue campaigns are fundamental for the conservation of the Montagu's Harrier population of south-eastern Spain.—Moreno-Rueda, G., Lopezosa, P. & Rivas, J.M. (2019). Breeding biology of Montagu's Harrier Circus pygargus in south-eastern Spain. Ardeola, 66.1: 3–11.