Mizar, a proof-checking system, is used to build the Mizar Mathematical Library (MML). This is a long term project aiming at building a comprehensive library of mathematical knowledge. We describe issues concerning information retrieval, i.e., searching, browsing and presentation of MML contents. A web-based tool providing such functionalities is being implemented by G. Bancerek. We hope that our observations are helpful when solving similar problems for other repositories of formalized mathematics.