This report, which has been prepared as part of the IAEA programme on radioactive waste disposal, discusses the approaches used in the hydrogeological investigation of repository sites. It is based on experience gained in Member States on those rock types considered as having the potential to host a repository. Contents: 1. Introduction; 2. Geological disposal of radioactive waste; 3. Function of the geological barrier in a disposal system; 4. Focusing hydrogeological investigations; 5. Hydrogeological issues in disposal systems; 6. Approaches to characterization of the hydrogeological system; 7. Summary.