Abstract
<jats:p>Tax administrations are at the forefront of technological adoption, driven by the promise of greater efficiency in the application of the tax system and by the need to strengthen the fight against increasingly complex forms of fraud. Yet this technological transformation raises profound legal challenges. It places us before a central tension of our time: the collision between the legitimate pursuit of administrative efficiency through algorithmic tools and the indispensable protection of taxpayers’ fundamental rights and guarantees within the framework of the rule of law. This monograph addresses that tension by systematising and classifying the artificial intelligence systems used by the Spanish State Tax Administration Agency, based on an unprecedented empirical analysis of public procurement during the 2015–2025 period. This technological reconstruction shows that algorithmic control has extended to multiple areas of the Agency’s activity, revealing imbalances between official discourse and operational reality, and between administrative powers and citizens’ rights. In response, the book argues for the urgent need to develop a substantive algorithmic legality, grounded in proactive transparency and explainability, capable of ensuring that the legitimate pursuit of tax collection efficiency does not undermine human dignity or reduce citizens to mere statistical objects beyond effective judicial control.</jats:p>