It has been widely noted that China and Russia have grown progressively closer over the last two decades. Although the scholarly literature has offered detailed descriptions and various ad hoc explanations of this trend, the Sino-Russian bilateral relationship has been the subject of very little scrutiny using rigorous theory, which has obstructed hypothesis formation and evaluation. Moreover, the cooperative post-Cold War trend in the bilateral relationship seems puzzling for baseline versions of each of the major paradigms of international relations theory: realism, constructivism and liberalism. For realists, China's rising power, coupled with its geographic proximity and longstanding border disputes with Russia, made it a present and growing threat to Russian security at the end of the Cold War. Why did China's rise not incur balancing from Russia and increasing bilateral hostility, rather than reconciliation? For constructivists, the stark differences in political ideologies and national cultures, as well as a long history of antagonism, presaged continued post-Cold War animosity. How have these historical animosities and ideological rifts been mitigated or overcome? Finally, both countries were increasingly integrated into the US-led international order immediately following the Cold War, with relatively low interdependence in their bilateral relationship. From a liberal perspective, why did this not prompt the two countries to improve political relations with the West while holding each other at arm's length? The papers in this special issue develop and apply nuanced theoretical arguments to derive testable hypotheses for the cooperative trend in China-Russia relations. In contrast to existing scholarship, these papers offer generalizable insights that both improve our understanding of a crucially important contemporary case, while also advancing IR theory in substantial ways.