a systematic reintroduction scheme including consecutive supplementations made it possible to infer success of reintroduction, supplementation and following admixture of populations. 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 of populations, genetic diversity and also effective population size in the wild stabilized or even increased. Multivariate (DAPC) and Bayesian inference (STRUCTURE) revealed admixture of supplemented individuals with wild-born individuals. Although population size estimates differed strongly between populations, a link between census size, breeding lines, effective population size and genetic diversity could not be proven. This study highlights that genetic monitoring is not only descriptive but also reveals detailed information on reintroduction success, admixture and population development. We recommend that genetic monitoring should be a basic element of reintroductions and should be used to optimize reintroduction attempts.Keywords Captive breeding · Common hamster · Founder effect · Admixture · Genetic variation · Effective and census population size IntroductionThe current global biodiversity crisis affects many groups of species, including a wide range of mammalian species (Di Marco et al. 2014). A global conservation strategy for mammalian species is therefore urgently needed (Rondinini et al. 2011), but the success of conservation strategies is uncertain as many factors influence the result of conservation projects (Crees et al. 2016). The ultimate strategy to prevent extinction or to increase population Abstract The ultimate strategy to prevent species extinction is captive breeding followed by reintroduction of individuals into the wild. Unfortunately, overall success of reintroductions is poor and in most cases conservation breeding is applied for species where individual numbers, population numbers and genetic diversity is strongly reduced. In addition, reintroductions inevitably result in small populations with poor genetic status. Systematic demographic and genetic monitoring is needed to optimize conservation actions. 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. Using well defined breeding lines of original relict populations combined with M. J. J. La Haye and T. E. Reiners have contributed equally to this work. numbers of mammalian species is captive breeding followed by reintroduction of captive-bred individuals into the wild (IUCN 2013). However, captive breeding and reintroduction have severe disadvantages. Reintroductions can be difficult and expensive (Tenhumberg et al. 2004), and the release of captive-bred individuals is often accompanied by high initial losses of released individuals (Villemey et al. 2013;McCleery et al. 2014). Moreover, captive breeding is mostly started in species with already highly threatened popu...