Geological repositories for the disposal of high level radioactive waste are designed to provide isolation of the waste from the human environment for many thousands of years. Monitoring the environment of such repositories is not therefore expected to reveal any increase of radioactivity due to the repository; it is planned and intended for other purposes. These include providing reassurance that the repository is fulfilling its intended purpose, but there can be other reasons of a technical nature. This report discusses the possible purposes for monitoring geological repositories at the different stages of a repository programme, the use that may be made of the information obtained and the techniques that might be applied.