Sweden is the only state in the Baltic Sea region currently not under NATO protection, which puts the state in a particularly vulnerable position. On 18 May 2022, Sweden officially deemed NATO alignment a more effective option of security strategy in projecting deterrence and dealing with the security challenges the state is facing, compared to a strategy of non-alignment which has been Sweden’s security posture for over 200 years. The aim of the thesis is to explain this shift in Sweden’s security policy.  The thesis will explain small states’ security policy beyond the traditional explanations found in realist theory of state-centric threat balancing and sovereignty. Shelter theory claims that small states’ options for security is either to find a protecting power or join an alliance in order to be politically and militarily sustainable. The thesis will analyse Sweden’s shift in security policy by examining shelter theory’s claim that in order for small states to survive and prosper, buffering up domestic capabilities does not suffice, they need to seek political and military shelter from external security providers by implementing bi- or multilateral agreements with neighbouring states, great powers and by joining alliances.  Analysing a long-time deviant case of a non-aligned small state, this thesis argues that Sweden’s drawn-out road-map to NATO membership is problematic to explain from the perspective of shelter theory. Consequently, the thesis makes the additional claim that factors related to Sweden’s domestic policy, such as public opinion and the nearly institutionalised practice of broad political consensus in issues relating to security policy, also play an important role for the design of Sweden’s security policy. Shelter theory in combination with domestic factors’ influence on security policy change offer a more fully fledged explanation of Sweden's shift in security policy to NATO alignment. This thesis argues that its findings complement and enhance shelter theory by shedding light on the importance of domestic factors in the study of small states’ security policy.