This comprehensive Handbook presents an overview of the nature and role of firms in contemporary economies. Leading scholars offer a multidisciplinary perspective that integrates classic and contemporary theories, demonstrating how foundational ideas have evolved in response to globalization, digitalization, and shifting societal expectations. Moving beyond a narrowly economic perspective, the Handbook conceptualizes the firm as a multi-dimensional institution embedded in complex environments. Contributions address core themes such as economic coordination, organizational design, and legal structure, while also engaging with issues of social and political legitimacy. Rather than advancing a single unified model, the volume positions the 'Theory of the Firm' as a broad and evolving research agenda for understanding contemporary organizational forms. It highlights the analytical value of pluralism and the necessity of interdisciplinary approaches to capturing the complexity of modern firms. The Handbook on the Theory of the Firm is an essential resource for scholars and students in economics, strategic management, international business, corporate law, and governance. It will also be of interest to policymakers and public-sector researchers concerned with how firms interact with markets and with legal, political, and societal environments.