Large knowledge bases are being developed to describe entities, their attributes, and their relationships to other entities. Prior research mostly focuses on the construction of knowledge bases, while how to use them in information retrieval is still an open problem. This paper presents a simple and effective method of using one such knowledge base, Freebase, to improve query expansion, a classic and widely studied information retrieval task. It investigates two methods of identifying the entities associated with a query, and two methods of using those entities to perform query expansion. A supervised model combines information derived from Freebase descriptions and categories to select terms that are effective for query expansion. Experiments on the ClueWeb09 dataset with TREC Web Track queries demonstrate that these methods are almost 30% more effective than strong, state-of-the-art query expansion algorithms. In addition to improving average performance, some of these methods have better win/loss ratios than baseline algorithms, with 50% fewer queries damaged.