This chapter examines impact assessment (IA) as an important tool for guiding sustainable investment. It starts by defining IA and discussing its historical development. This chapter highlights its increasing role in assessing the environmental, social, and economic effects of financial choices. It introduces key IA frameworks, such as life cycle assessment, cost-benefit analysis, and risk management, while examining important values and metrics for measuring impact. This chapter also points out challenges in applying IA and considers its future possibilities. It further discusses the processes of monitoring, evaluation, and reporting, emphasising best practices and common mistakes. Through case studies of organisations like the IFC, EBRD, World Bank, and Intel, it shows how IA applies in the real world. Finally, it moves into sustainable investing, differentiating between socially responsible investing, ESG investing, and impact investing as ways to connect finance with long-term social goals.