Pastoralist societies have developed customary institutions to respond to an unpredictable environment and fluctuation in grazing resources for their livestock. This chapter describes how reindeer herders’ customary institutions, including laws, norms and rights embedded in social networks, as well as traditional knowledge, structure these responses. Furthermore, it analyses how reindeer herders’ customary institutions are integrated into state governance of natural resources or recognized in national legislation. Central to the chapter is the Sámi siida and the corresponding Finnish tokkakunta – both represent customary herding groups that seek to balance the relationship between human–reindeer units to the spatial and temporal availability of grazing resources. The need for revitalization and a better understanding of reindeer herders’ customary institutions is identified, as well as an increased recognition of their traditional knowledge in resource management and land use planning to increase the resilience of reindeer husbandry to the cumulative challenges of climate change and resource extraction.