The deep sea (>200 m depth) encompasses >95% of the world's ocean volume and represents the largest and least explored biome on Earth (<0.0001% of its surface). It also provides critical climate regulation and other ecosystem services. New species and ecosystems are continuously being discovered in the deep oceans, but commercial fisheries, deep-sea mining, and offshore oil and gas extractions, along with pollution and global change effects, threaten this vast under-explored frontier region. The future of both benthic and pelagic deep-sea ecosystems depends upon effective ecosystembased management strategies enhancing deep-sea conservation, yet we lack consensus on monitoring of the biological and ecological variables that reflect ecosystem status and are needed to support management and environmental decisions at a global scale. Here, we present and discuss the results of an Expert Elicitation of more than 110 deep-sea scientists to prioritize variables and parameters for the future of deep-sea monitoring. We identified five main scientific pillars that need to be further investigated for deep-ocean conservation: i) species and habitat biodiversity, ii) ecosystem function; iii) ecosystem health, impacts, and risk assessment; iv) climate change impacts, the adaptation and evolution of deep-sea life, and v) deep-sea ecosystem conservation. As observing and monitoring can provide the necessary scientific framework for scientists and policy makers to implement effective deep-sea conservation strategies at a global scale, the proposed variables should be further studied in the context of available sensor and other advanced technologies, which are becoming increasingly available.