The examination of bridges under service loading is key for understanding their condition and thus for their sustainable and efficient use. The need to extend the service duration of existing bridges is increasing, and therefore, efficient tools for examining and maintaining them are required. Monitoring of structures is recognised as an efficient method for investigating structural behaviour and evaluating structural performance. However, there is a lack of methods for choosing suitable sensors and performing monitoring on-site. The main challenges associated with monitoring include environmental conditions, complex loading, big data storage, and measurement calibration. Thus far, no research has developed and implemented a complete strain-acoustic-emission monitoring method, from instrumentation to data interpretation, to evaluate long-term changes in the internal state of reinforced-concrete road bridges. The general objective of this thesis is to characterise fatigue-related damage in the reinforced concrete of bridge-deck slabs using nondestructive methods. The influence of low fatigue stress on reinforced-concrete behaviour was studied at a microscale structural level, and this research was then used to define two indices that quantify existing damage and fatigue-damage increase over time. For this aim, an extensive long-term monitoring campaign was conducted on a bridge in service to understand the fatigue behaviour of reinforced-concrete slabs under operational loadings and to evaluate the effectiveness of different nondestructive methods. The instrumentation and calibration of the used nondestructive measurements produced recommendations for the examination of reinforced-concrete bridge-deck slabs. The methods provide a solid understanding of how to quantify the influence of traffic loading and environmental changes on structural behaviour, such as fatigue behaviour, and structural characteristics such as the connection between bridge components. The results derived from the monitoring data indicate that the instrumented bridge is safe with respect to the fatigue limit despite the potential difference between the present and original design conditions. The developed inverse method identifies the probabilistic distribution of the position and the load of vehicles crossing the instrumented slab. The results of this method define the link between the fatigue-damage distribution and the position and the load of vehicles, and reveal the sensitivity of fatigue damage to different sources of uncertainties associated with the compressive strength of concrete, the level of stresses, and the magnitude of annual traffic. Combining information from monitoring data and structural analysis models allows for accurate data interpretation and model development. The results reveal that the identification of structural parameters and the prediction of fatigue endurance of the noninstrumented structural elements can be performed accurately. Integrating acoustic emission and strain measurement results is a comprehensive method for analysing the fatigue of reinforced-concrete members by which microcracks in the concrete volume can be localised and visualised under operational loading. The stationary movement of existing microcracks and low microcrack growth govern the concrete’s behaviour under low fatigue stresses. Reinforced concrete was found to be in the stage of new-microcrack nucleation and stable microcrack growth.