In the 1930s, Maxim Litvinov, Soviet foreign minister under Joseph Stalin, proposed the principle of the indivisibility of peace as one of the principles with which all nations were expected to comply to maintain peace among them. The principle implied that no nation could be at peace when another nation was not at peace. In the 2010s, Sergey Lavrov, Russian foreign minister under Dmitry Medvedev and Vladimir Putin, proposed a new security treaty for a wider Europe based on the principle of indivisible security. The latter principle maintained continuity with the pre-WWII principle of the indivisibility of peace; it implied that no nation could be secure when the security of another nation was compromised. Simultaneously with discussing the implications of the principle of the indivisibility of security for Russian foreign policy, this chapter highlights the process of incorporating of the concept of the indivisibility of security into theoretical debates among Russian IR scholars. It invokes Lakatos’s methodology to provide insight into attempts to incorporate the concept of the indivisibility of security into established research programs dominated by concepts borrowed from Western IR theory: namely, those of collective, common, and cooperative security.