This book aims to show a map of tax decentralization that allows framing and evaluating the Spanish experience, the difficulties and pending challenges, especially at the regional or autonomous level. But rather than giving an unequivocal solution, the aim is to put it in perspective, to know what has worked well and badly in other countries, to determine in which aspects the Spanish model is in the average and in which it is an anomaly or a particular case; in short, to delimit the space of alternatives for political agreement and for the legislator.