This document discusses various efficiency theories for mergers and acquisitions. It outlines theories such as differential efficiency, inefficient management, synergy, pure diversification, and strategic realignment. It also discusses undervaluation as a factor for M&As. Additionally, it covers agency problems that can arise between managers and shareholders, and how acquisitions can help address these issues. Finally, it outlines the hubris hypothesis, where managers may commit errors of overoptimism when evaluating merger opportunities due to excessive pride.