Biological diversity, or biodiversity, refers to the variety of life forms and their interactions within ecosystems. Defined by the Convention on Biological Diversity, it includes variability among living organisms from various sources, encompassing diversity within species, between species, and of ecosystems. Biodiversity is essential for ecosystem functions and also provides vital services to humans, such as clean air and water, food production, wood and other natural resources, or climate regulation. It is also linked to the United Nations’ Sustainable Development Goals (SDGs), impacting health, water management, climate action, and food security. Understanding and preserving biodiversity is crucial for maintaining ecosystem services and promoting a sustainable future. This chapter explains the principal theme of the book—biodiversity—from the biological point of view, points out its immense importance for functional processes on our planet and argues in favor of nature-based school education as an important contribution to counteract people’s decreasing awareness for the necessity for the protection of Earth’s biodiversity.