This introductory chapter sets the conceptual and methodological foundations of the book Innovation Systems from Below. It presents an actor-centred approach to the study of innovation systems, emphasising how firms and intermediaries build capabilities, respond to constraints, and shape their institutional environments. Drawing inspiration from systems of innovation theory and complementary perspectives on innovation in the Global South, the chapter situates the book within a growing effort to theorise innovation from the ground up. It outlines the rationale for the Innovation Systems from Below (ISFB) framework, which integrates insights from political economy, economic geography, and development studies to analyse innovation as a relational, context-specific process. The chapter also introduces the methodological and empirical foundation of the book, which is based on fieldwork conducted in Brazil and Mexico involving six organisational observations across public and private sectors, and describes the analytical strategy used to interpret the data.