This chapter revisits the Systems of Innovation (SI) approach and proposes a novel actor-centred framework, the Innovation Systems from Below (ISFB), to analyse innovation in diverse economic contexts. While the SI model has shaped scholarly and policy debates around national innovation systems, its assumptions often falter in developing economies characterised by weak institutions, informal networks, and fragmented absorptive capacities. Building on critiques from innovation studies, political economy, and economic geography, this chapter reconfigures the SI lens by integrating global linkages, socio-spatial dynamics, and firm-level agency. It introduces key concepts such as strategic coupling and institutional bridging to explain how innovation emerges through both formal and informal mechanisms. Grounded in mechanism-based explanation, the ISFB framework shifts focus from system-wide aggregates to the situated practices and strategic actions of firms, universities, and intermediary organisations. The chapter also engages with global innovation networks, regional development, and the geography of knowledge to offer a comprehensive analytical tool that addresses both micro- and macro-level dimensions of innovation.