In this report we provide the theoretical foundation for the concept of Nature Intelligence. It consists of four parts. In the first part we give an overview of theories on the concept of intelligence in general, and theories on concepts related to Nature Intelligence. In the second part, we present a conceptual model of Nature Intelligence based on theoretical insights and hands-on-experience from youth organisations. In the third part we describe how we developed a scale to measure Nature Intelligence. Finally, in the fourth part we discuss insights from research on nature-based education that shed light on the critical success factors for promoting Nature Intelligence. This report is available in English only.