The long-term success rate of dental implants has been considerably increased by the osseointegration concept and in the last decades, a lot of different implant systems have been developed to replace missing natural teeth. The predictable long-term success of osseointegrated dental implants seems to result from the determination of specific problems correlated with the system itself and the identification of factors which can affect the success of therapy: these considerations lead to the conclusion that monitoring each patient is absolutely necessary. In the paper presented, the various clinical parameters habitually used in the evaluation of dental implants during the maintenance phase of therapy are reviewed. The parameters which are discussed include probing and attachment level measurement, the evaluation of inflammation indexes, mobility, suppuration, tissue temperature, radiology and microbial monitoring. Unfortunately, many clinical signs of failure appear only when an irreversible stage has been reached: for example, when mobility is detected, we know that osseointegration, defined as "a direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant" by Albrektsson and Brånemark has been lost. According to the fact that little evidence of the etiological and pathogenetic mechanisms implant failure exists, great attention should be given to the establishment of a comprehensive preventive, diagnostic and therapeutic approach to the problem.