Ecosystem-based adaptations (EbA) are nature-based solutions that include a broad array of ventures for the management of ecosystems, such as environmental protection, restoration of ecosystems, and activities to lower the vulnerability and improve the resilience of communities under a changing climate. In fact, adaptation measures can tackle the harmful aspects of climatic variations as well as assist vulnerable communities in preparing to face climatic changes in the long run. The concepts of EbA and resilience are inherently linked as they both address the capacity of ecosystems and communities to absorb disturbances and still be able to bounce back. EbA and resilience depend on thresholds, which are boundaries around a system state. If crossed, these thresholds represent a transition to a new system with a new set of components and relationships. The system’s response will also depend on its adaptive capacity (i.e. the capacity to learn, cope, innovate, and adapt), which in turn is determined by the amount and diversity of social, economic, physical, and natural capital, as well as social networks, institutions, and entitlements to the distribution and use of such capital. The implementation of EbA can be envisaged independently or combined with engineered measures (e.g. the raising of dykes of reservoirs of water), blended approaches (e.g. artificial reefs), and measures that build up the capability of communities as well as institutions to adapt to climatic risks (e.g. the introduction of early warning systems). Therefore, effective implementation of EbA should include transformative action involving both climatic and non-climatic drivers of change. In this chapter, we define the concepts and principles of EbA and evaluate its application, using lessons learned from mountainous and hilly ecosystems worldwide.