The vast deep sea is the largest living space on earth and yet less than 5% of the seafloor has been explored in some details. In fact, we have better maps for the Moon, Mars, or even Venus due to our planet’s watery veil. The variety of habitats and the complex relationship with its inhabitants gives rise to diverse communities, providing important ecosystem functions and services, and maintaining the elemental cycling in the ocean. However, climate changes and human activities on the seafloor have created unprecedented challenges and threats to the deep-sea ecosystems. These seemingly pristine environments have undergone major changes that out-pace our understanding of factors that drive the spatial and temporal patterns of the deep-sea ecosystems. This course will cover major topics in deep-sea ecology including the physical environments and history, sampling techniques, adaptations of deep-sea organisms, community composition and functions, major habitats, and anthropogenic effects.