An ontology is a computer-processable collection of knowledge about the world. This thesis explains how an ontology can be constructed and expanded auto- matically. The proposed approach consists of three contributions: 1. A core ontology, YAGO. YAGO is an ontology that has been constructed automatically. It com- bines high accuracy with large coverage and serves as a core that can be expanded. 2. A tool for information extraction, LEILA. LEILA is a system that can extract knowledge from natural language texts. LEILA will be used to ¯nd new facts for YAGO. 3. An integration mechanism, SOFIE. SOFIE is a system that can reason on the plausibility of new knowl- edge. SOFIE will assess the facts found by LEILA and integrate them into YAGO. Each of these components comes with a fully implemented system. Together, they form an integrative architecture, which does not only gather new facts, but also reconcile them with the existing facts. The result is an ever-growing, yet highly accurate ontological knowledge base. A survey of applications of the ontology completes the thesis.