Welcome to the Islands Clean Energy Transition Handbook. This Handbook is an action- oriented guide to help islands navigate the transition toward clean energy. It follows a central methodology (EXPLORE, SHAPE, ACT) to cover ideas and projects in all stages of the clean energy transition. It also provides examples of European islands and includes references to key publications. The EXPLORE part addresses islands that are just starting their clean energy transition. It guides the development of a Clean Energy Transition Agenda (CETA), i.e., a strategic roadmap to decarbonise the island. The SHAPE part aims to bring forward the potential solutions suggested in the CETA to shape them into a pipeline of bankable clean energy projects. The project idea needs to be first developed, followed by studies assessing the feasibility of the projects, and the development of the business case. The ACT part focuses on further developing the projects so that they are ready to be implemented on the ground, including finding the right funding and financing, developing the business plan, and setting up the required partnerships for implementation.