Thomas Aquinas (1224/25-1274) has led the philosophy and theology of Latin scholasticism to an exemplary synthesis in his extensive work, which has remained influential to this day. In his treatise "De ente et essentia," written early in his career, he presents the logical-ontological fundamental concepts of his philosophy and outlines their systematic significance for his metaphysics. This short but dense treatise is rightly regarded as the best introduction to this core aspect of his thought.