Both scholars and decision-makers recognize that the functioning of an organization is the result of a complex interplay between purposively designed elements, such as structures and processes, and emergent elements, like the informal networks of relationships that connect individuals and groups. Despite this widely shared awareness, scientific knowledge on this relationship and its translation into coherent models and principles of organizational design remain fragmented and unstructured. This book adopts a network perspective that conceptualizes organizational structure as a landscape of interconnected networks composed of actors' interactions and relationships. Some of these networks are intentionally designed, grounded in formal structures, processes, and workflows. Others emerge informally, shaped by instrumental and expressive social ties. Together, they constitute the relational infrastructure through which actors coordinate and access key resources, including information, knowledge, and social or political support. Drawing on empirical studies conducted across diverse organizational contexts, the book develops a unified theory and methodology for interpreting and understanding the complex relationship between formal organizational structures and informal social networks, how they shape one another, evolve together, and jointly influence organizational outcomes