This guide outlines the process of building a lean business case, which is essential for evaluating business opportunities in a structured and efficient manner. It emphasizes the importance of innovation while managing risks and uncertainties associated with investment decisions. The document details the lean business case methodology, which includes creating a comprehensive yet concise view of potential business opportunities. It describes the iterative nature of developing a business case, highlighting the need for speed, efficiency, and data-driven visual storytelling. The guide also presents the four key elements typically found in a lean business case: initial business models, early solution drafts, financial models, and analysis with recommendations for next steps. Additionally, it discusses the significance of documenting assumptions and the iterative process of validating them through minimum viable products (MVPs). The guide serves as a resource for business developers, product managers, and innovators looking to make informed investment decisions.