In the post-closure phase of a deep geological repository for radioactive waste, significant quantities of gases may be generated from various sources. The most important gas generating process is anaerobic metal corrosion. Additionally, degradation of organic matter by bacterial activity and water radiolysis contribute to gas formation. Non-radioactive gases can be important because of potential pressure related impacts on the engineered barrier system and geological host formation, and their role as a carrier gas for transporting radioactive gases. Repository designer may thus face a possible conflict of goals: while the design strategy of a disposal system is to provide isolation and containment, the non-radioactive gases may need to be dissipated to minimise any adverse effects from pressure build-up in the repository system.