Safety-related investment decisions for tunnels rely on evidence of the benefits of risk reduction measures and when risks are considered acceptable. These decisions can be at the level of a policy across an organisation or a specific investment decision for a tunnel project. Across different sectors, established relevant good practice is used as a baseline for what will result in an adequate level of safety. Where relevant good practice is appropriate to the circumstances, then decisions on risk mitigation are relatively straightforward. Where established good practice does not exist, or is out of date, the decision making process is more complex. This article first presents a review of safety risk acceptance principles for UK tunnels, comparing principles adopted for rail tunnels, road tunnels, tunnels subject to buildings regulations, and tunnels in nuclear installations. Case studies are presented from each of these sectors. As more tunnels are constructed, including in densely populated cities, it is increasingly important to understand the grounds for decision making. Efficient allocation of resources needs to be weighed alongside uncertainty, loss prevention and the foreseeable impact of major tunnel incidents. This article identifies a lack of clear guidance on risk acceptance principles for tunnels, and makes recommendations for the factors that should be addressed by tunnel specific guidance. We highlight societal concerns, which are a significant factor in decision making over tunnel safety risks.