This GeoGuide provides an overview of the geology of Alnö, combined with an up-to-date field itinerary. Covering all major geological aspects, it offers an essential summary of Alnö and its intriguing magmatic rocks in a compact form suitable for field excursions and home study alike. As one of the type localities for carbonatite, the late Proterozoic Alnö ring complex has been a crucial site for carbonatite-related research (next to the Fen complex in Norway), and provided one of the earliest test beds for this unique group of igneous rocks. Five geological excursions introduce the visitor to the most rewarding outcrops, including detailed descriptions and a wealth of high-quality colour photographs. The excursions are complemented by a detailed review of the history of scientific investigation on Alnö and, in particular, a catalogue of exotic and common minerals associated with the complex’s carbonatitic and alkaline silicate rocks. Finally, a summary of its trace element and isotope geochemistry as well as a brief outlook on Alnö’s potential as a future source of Rare Earth Elements (REEs) completes the book.