Reintroductions have become more popular to prevent species from extinction in the wild. However, reintroductions inevitably result in small populations with poor genetic status. Systematic demographic and genetic monitoring is needed to optimize conservation actions like reintroductions. Here we show how genetic monitoring was useful and informative in a reintroduction project for the highly endangered Common hamster (Cricetus cricetus) in the Netherlands and Belgium. An initial loss of genetic diversity could be detected in some of the reintroduced populations, but it could be shown that due to following supplementation, extra releases in established populations, genetic diversity and also effective population size in the wild stabilized or even increased. Samples for the DNA analysis in this study were collected within the conservation and breeding program on a regular basis since 1999 and in the wild whenever possible, including tissue from dead hamsters. All samples were genotyped for a maximum of eight polymorphic microsatellite loci: Ccrμ10, Ccrμ11, Ccrμ12, Ccrμ15, Ccrμ17, Ccrμ19, Ccrμ20 (Neumann & Jansman 2004) and Cricri-IPK02 (Jakob & Mammen 2006). All loci had only two different alleles. In our study we used 818 genetic profiles, of which 268 profiles are from wild-sampled individuals and 550 from individuals of the captive breeding program that were released into the wild. The individuals that were used for reintroduction were exactly known due to the Studbook of the European hamster which is managed by Rotterdam Zoo (de Boer 2014). However, not all genetic profiles of released individuals were available and missing genetic profiles were inferred by reconstructing the total pedigree, with known genotypes from founders and their descendants as input. The file Hamster_Genotypes_200520_csv has the following columns: Column 1 AREA = (populations in the Netherlands: NL1, AMBY, HEER, SIBBE, PUTH, SITTARD, populations in Belgium: BERTEM, TONGEREN, population in Germany: GER1) Column 2 Time/Period = ...