Reindeer pastoralism is a nomadic circumpolar livelihood involving more than 20 different indigenous peoples around the entire circumpolar north. The livelihood involves approximately 100,000 people and 2,5 million semi domesticated reindeer. This thesis is a comparative study of the social organization in two reindeer herding regions: the Yamal-Peninsula of Western Siberia, and Western-Finnmark of Northern Norway. This thesis investigates how resilience to climate change is embedded in the social organization of reindeer pastoralism, and further how non-climate factors in terms of institutional constraints and opportunities affect resilience. The thesis shows that despite climatic differences and differences in the institutional setting, the social organisation of Nentsy pastoralists in Yamal and Sámi pastoralists in Finnmark have strong structural similarities, and further that the social organisation has mechanisms for securing resilience, as the organisation allows herders the freedom to determine the structure and size of the herd according to available natural resources and to determine the best strategy for mobility and migration. Further the thesis argues that the different institutional settings in the two regions are responsible for different effects on the resilience of Nentsy and Sámi reindeer pastoralists.