The concept of minimising life cycle cost can be rationally applied to determine a target reliability of structures. However, there has been a recent legal requirement to reduce CO2 emissions, therefore, the minimisation of life cycle CO2 emission could be an alternative approach in structural design. This paper explains the general formulae for deciding the optimum reliability by minimising the total life cycle cost and the amount of CO2 emission in the structure's lifetime, and also shows that these formulae can be applied to decision making for the cost-benefit problem related to seismic strengthening of structures. Additionally, the role of the engineer is discussed, reflecting the current situation in Japan.