Interorganizational networks are formal or informal relationships between organizations established to share knowledge, resources, and expertise with the intent to create value and, possibly, achieve a common goal. In today’s complex business environment, forms of collaboration and relationships, such as joint research and development, supply chains, and social and environmental initiatives, require careful planning and ongoing effort to match the desired targets. The attention of management scholars is increasingly directed towards the complexity of such interorganizational networks, where heterogeneous firms form different types of ties among each other—possibly at the same time—giving rise to multiplex structures. Given the inherent complexity of interorganizational networks, the evolution of such structures in time and the achievement of their objectives also rest on the processes underpinning network formation. This chapter analyses different approaches to network formation, discussing their impact on shaping emerging structures. It also discusses how agent-based models can contribute to the modelling of complex network formation.