Sea slugs mesmerize with their unparalleled beauty, boasting extravagant colours and unique body shapes. Their magnificence draws the attention of divers and underwater photographers from across the globe, who eagerly seek out and document these remarkable creatures. Beyond their aesthetic appeal, sea slugs exhibit remarkable biology and evolutionary processes, showcasing impressive regenerative abilities and producing unique chemical compounds for self-defence in the absence of protective shells. Some of these compounds have even demonstrated promising anti-cancer properties, positioning sea slugs as valuable subjects in biomedical research. Situated midway between the American and European continents, the Azorean islands are a living laboratory where species from both sides of the Atlantic coincide, while at the same time, the geographical isolation of the islands has fostered unique biodiversity and the evolution of endemic species found nowhere else on Earth. In an era where climate change and invasive species pose significant threats to local faunas, a book like this not only serves as a tool to document the marine biodiversity of the Azores but also as a means to raise awareness among the public, nature enthusiasts, and policymakers. It encourages a deeper understanding and respect for our natural heritage, fostering a collective effort to conserve and protect it.