Implant stability and maintenance of stable crestal bone level are prerequisites for the successful long-term function of oral implants, and continuous crestal bone loss constitutes a threat to the longevity of implant-supported prosthetic constructions. The prevalence/incidence and reasons for crestal bone loss are under debate. Some authors regard infection (i.e. peri-implantitis) as the cause for virtually all bone loss, while others see crestal bone loss as an unavoidable phenomenon following surgery and implant loading. Irrespective of the cause of continuous crestal bone loss, correct usage and scientifically sound interpretation of radiographs are of utmost importance for evaluation of oral implants. The periapical radiographic technique is currently the preferred method for evaluating implant health based on bone loss, and digital radiographs allow easy standardization of the image contrast. It is suggested that baseline radiographs should be taken at the time the transmucosal part pierces the mucosal tissues and annually thereafter. The number of unreadable radiographs should be presented in scientific publications to give insights into the quality of the radiographic examination. It is suggested that not only mean values, but also the range of bone levels, should be presented to describe the proportion of implants that show continuous crestal bone loss. In the absence of other clinical symptoms, bleeding on probing around implants seems to be a weak indicator of ongoing or future loss of crestal bone. According to recent longitudinal studies on modern implant surfaces peri-implantitis defined as 'infection with suppuration associated with clinically significant progressing crestal bone loss' occurs with a prevalence of less than 5 % in implants with 10 years in function.