<p>Across its entire range, hazel dormouse (<i>Muscardinus avellanarius</i>) populations are increasingly showing signs of decline. In addition, due to insufficient long-term capture-mark-recapture studies, little is known about hazel dormouse population dynamics, making it difficult to target specific conservation measures. In this study, we used long-term capture-mark-recapture techniques to identify demographic patterns of the hazel dormouse population over a period of 40 years and with 26 years of continuous monitoring. By fitting Hidden Markov Models to the annual population growth rates, we identified four distinct periods of population development, including decline, increase, stable high and stable low abundance. Each period lasted from five to eight years and varied in their demographic characteristics. In particular, the number of overwintered females appeared to shape the population dynamics. Our study shows how the hazel dormouse population declines from maximum spring population density at 1.4 individuals per hectare during stable high abundance period to minimum 0.4 individuals per hectare at stable low abundance period and stabilizes at low level for several years without disappearing. Further studies are needed to investigate which extrinsic and intrinsic factors drive changes in population development.</p>