Mountains are home to a variety of ecosystems that provide vital services directly to 1.1 billion people and billions of others living in connected lowland areas. Half of humanity depends on mountains for the provision of freshwater alone. Mountain ecosystems cool local temperatures, increase water retention, provide carbon storage, and reduce the risk of erosion and landslides. Mountain forests, wetlands and grasslands also host and support half the world’s biodiversity hotspots. But the world’s mountain ecosystems are under attack due to their particular sensitivity to the planetary crises of climate change, biodiversity loss, food insecurity, and pollution and waste. Evidence shows that mountain ecosystems are affected at a faster rate than many other terrestrial habitats. This publication, jointly developed by the Mountain Partnership Secretariat at the Food and Agriculture Organization of the United Nations and the United Nations Environment Programme, analyses several mountain ecosystem restoration projects and recommends how the UN Decade’s Ten Principles for Ecosystem Restoration can be applied to mountain ecosystems. Mountain restoration success stories from initiatives that have been selected or shortlisted as the UN Decade’s World Restoration Flagships are also highlighted. As the theme of International Mountain Day 2023 is “Restoring Mountain Ecosystems”, this publication provides an important contribution in addition to celebrating the Five Years of Action for the Development of Mountain Regions 2023–2027.