Could we analyze power-sharing through the rear-view mirror? The politics of Lebanon and Northern Ireland are relevant case studies to answer this question. Using Pierre Nora's lieux de memoire scheme of historical memory, this book crafts a theory of sites of social interaction (SSI) and strategies of social cohesion in power-sharing institutions. SSIs and cohesion strategies that increase tensions will cause power-sharing failure in the long run, and vice versa. Thus, there is a causal link between power-sharing and ethnic tensions in divided societies, through mechanisms of SSIs and cohesion strategies. Lebanon and Northern Ireland encode power-sharing with different sites of social interaction and cohesion strategies, as a reflection of a society's composition and institutional design, respectively. Power-sharing implementation provides the missing link in our knowledge of power-sharing and ethnic tensions. At Boston College, this work won the 2021 Donald S. Carlisle Award for academic excellence in political science.