Before establishing the priority settings for the reduction of seismic risk of water supply infrastructures, it is necessary to understand the dynamic behavior of elevated water tanks, which are components of those infrastructures. Among other information, the main frequencies of vibration of these structures must be estimated and the analytical models used in their analysis and design should reproduce the frequency values obtained by in-situ dynamic tests. This work focuses exclusively on reinforced concrete (RC) elevated water tanks (200–750 m^3 of water at heights of 30–40 m), which are very common structures in the water supply systems in Portugal since the mid XXth century. This type of structures can also be seen in many regions around the world. First, a nationwide survey was conducted to determine the most common typologies in the country in terms of structural layout. Second, an in-situ campaign using ambient vibration as input was performed for a group of selected structures to determine the main frequencies of vibration and to identify modal shapes and damping values. Third, a finite element model of several different typologies was developed using the water simply as a concentrated mass at the top; the elastic properties of the model of the structure including the foundation were calibrated, so that the frequencies of various mode shapes obtained by the analytical model would match the frequencies of the real structure. Finally, an expression was derived to estimate the fundamental frequency of a group of elevated water tank typologies based on the total mass at the top of the supporting structure, which include the water, the global lateral stiffness, and the height of the tank. This study, providing important information on the frequencies of vibration of RC-elevated water tanks, contributes in a definite way to the analysis and design of such water tanks.