Design working life and target reliability levels recommended in various national and international documents are inconsistent. Indicative values of design working life are within a range from 10 to 100 years for different types of structures, recommended target reliability indexes are usually given for one or two reference periods (1 year and 50 years), without an explicit link to the design working life. The contribution attempts to clarify the relationship between the design working life and the reliability index and to provide guidance for specification of the target reliability level for given consequences, designs working life and discount rate. The theoretical study based on probabilistic optimization is supplemented by practical recommendations. It appears that the optimum reliability indexes depend primarily on the ratio of cost of structural failure (malfunctioning costs) and the cost per unit of structural parameter, less significantly on the design working life and discount rate.