<p>Apatite is a widely distributed accessory mineral in igneous and hydrothermal systems. Here we integrate cathodoluminescence (CL) imaging, in situ REE geochemistry, and oxygen and strontium isotope analyses of apatite from 43 samples spanning 16 European deposits representative of diverse magmatic to magmatic–hydrothermal settings, including carbonatites, alkaline complexes, lamproites, layered intrusions, IOA–IOCG deposits, granite-pegmatite systems, and hydrothermal and metasomatic veins. The combined dataset reveals systematic chemical and isotopic trends that distinguish primary magmatic apatite from magmatic–hydrothermal transition and hydrothermal–metasomatic generations, while also providing diagnostic criteria for different classes of phosphate-bearing deposits. Primary magmatic apatite is commonly characterized by LREE-enriched patterns, low δ¹⁸O values (generally 3–9‰), relatively low to moderate radiogenic Sr(i) signatures (0.701–0.707), and oscillatory CL zoning. Progressive fluid interaction with primary magmatic apatite is typically expressed by partial REE depletion, increasing δ¹⁸O values, more radiogenic Sr(i) compositions, and the development of dissolution–reprecipitation textures. Granite–pegmatite systems are distinguished by strong intra-REE fractionation expressed by tetrad effects (&gt; 1.1), radiogenic Sr isotopic compositions, and pronounced negative Eu anomalies. Apatite from hydrothermal and metasomatic deposits exhibits the strongest modification of primary signatures, including marked REE depletion, elevated δ¹⁸O values, radiogenic Sr(i) ratios, and commonly turbid or microcrystalline CL textures. This study therefore highlights the analytical and interpretative power of integrated in situ methodologies for resolving the evolution of economically and geologically significant P-rich systems.</p>