An ad hoc microcomputer based data acquisition and telemonitoring system, called ESSDI, has been developed by ENEL for dam monitoring. The main activities carried out on line by the ESSDI apparatus are: measurement and data acquisition, detection of particular facts, and dam behavior documentation. This documentation is made by sending data and alarm messages to a manned place, by recording data and alarms on cartridge magnetic tape and by showing information on a portable display unit. The authors describe the main objectives, the characteristics and the main features of the whole system. The prototype of the ESSDl apparatus is then described, by pointing out the main choices made in the design to guarrantee continuous correct information on dam behavior and to get a system that can easily be installed and operated by the plant personnel.