This comprehensive Companion synthesizes contemporary research on mergers and acquisitions (M&A), examining how corporate transactions reallocate vast resources, reshape industries, and influence firms, markets, and economic outcomes. The volume presents M&A as a central laboratory for studying value creation, corporate governance, strategy, regulation, contracting, and market efficiency. Leading experts integrate decades of scholarship with recent advances in the field, addressing foundational questions such as why firms merge, how value is created and distributed between stakeholders, how markets and regulators respond to acquisitions, and how mergers affect innovation, labour markets, and industry structure. The chapters combine diverse empirical approaches and theoretical perspectives, from classic event-study evidence to behavioural economics, cross-border institutional research, activist investors, and structural estimation, highlighting both enduring findings, including persistent gains to targets and the mixed performance of acquirers, and emerging research frontiers shaped by globalization, private capital, and data-driven methods. This Companionoffers researchers and students in business, economics, finance, and management an authoritative guide to what we know about mergers and acquisitions and where the field is heading. Practitioners and policymakers will likewise benefit from its insights into how M&A continue to shape corporate organization and economic activity in an era of technological and geopolitical change.