Contrary to widespread opinions, knowledge about durability is still more intuitional than approved by results of analysis. Durability of the structure depends on many factors, which can be grouped in various categories. Among them, the most important are: structural material, age and type of a structure, history of its operational use, types of loads affecting the structure, environmental factors and quality of maintenance. Those factors are estimated as a result of current, basic and detailed inspections or taking data from permanently used monitoring systems. The scores are reported in "records of bridges" and should be described numerically with coefficients. Using them is possible to create "degradation curves". Unfortunately, not sufficient number of numerically transformed data related to the system of estimation of technical condition of bridges still makes impossible creation of statistically valid "degradation curves" and correct values of coefficients. Observing the change of those estimations values in a longer period of time, it is possible to foresee tendency of changes and finally the durability of the bridge or its element and rationalize maintenance costs.