"Thinking Love: Heidegger and Arendt" explores the problematic nature of romantic love as it developed between Martin Heidegger and Hannah Arendt, whom Heidegger later called "the passion of his life." I suggest that three different ways of understanding love can be found at work in Heidegger and Arendt's relationship, namely, the perfectionist, the unconditional, and the ontological models of love. Explaining these different ways of thinking romantic love, this paper shows how the distinctive problems of the perfectionist and unconditional models played out in Heidegger and Arendt's relationship and how that relationship eventually gave rise to the third, ontological understanding of love. This ontological vision of love combines some of the strengths of the perfectionist and unconditional views while avoiding their worst problems, and so emerges as perhaps the most important philosophical lesson about romantic love to be drawn from studying the lifelong love affair between two of the twentieth century's greatest thinkers.