This chapter begins with a brief definition of biodiversity and its significance for humans. It then discusses the accelerating loss of biodiversity, characterising it as both an ecological crisis and a source of systemic financial risk. Recent evidence suggests an unprecedented decline in species, habitats, and genetic diversity, posing a threat to the stability and resilience of natural systems. The chapter introduces the principle of double materiality, which highlights how biodiversity loss affects businesses financially (referred to as outside-in) while also noting that businesses contribute to ecological degradation (referred to as inside-out) through practices such as land use change, pollution, and unsustainable resource use. In this context, banks play a crucial role: their lending and investment decisions can either exacerbate biodiversity pressures or help mitigate them. At the same time, the decline of ecosystems can negatively impact credit risk, collateral values, supply chain stability, and overall financial performance. Regulators and standard-setters increasingly recognise the necessity of integrating nature-related risks into financial governance and supervision. The chapter concludes by outlining the structure of the volume, which offers an interdisciplinary analysis of risk channels, banking exposure, measurement tools, and the implications of biodiversity loss for financial stability and bank behaviour.