Peri-implant diseases are diseases of infectious nature that affect the tissues surrounding a dental implant in function. Although periodontal and peri-implant diseases share many similarities there are also significant differences, especially regarding their treatment. The etiology of peri-implant diseases is closely related to the presence of a microbial biofilm on an implant surface, which consists of various bacterial species including specific periodontal pathogens, and to the existence of certain risk factors tied to the patient himself. Depending on which tissues are affected by the inflammatory process, peri-implant diseases are classified into peri-implant mucositis, in which the inflammatory process affects only the soft tissue surrounding an implant, and peri-implantitis, in which the process extends deeper into the connective tissue and leads to bone loss, compromising the osseointegration of an implant. Which tissues are affected by the inflammatory process is assessed during clinical examination, based on certain clinical diagnostic parameters. Without treatment, a peri-implant lesion will progress deep into the supporting bone and eventually result in implant loss. Treatment of peri-implant diseases can be non-surgical or surgical, depending on the severity and stage of the disease. In both cases, the most important steps of the treatment plan are: mechanical debridement of the implant surface and its decontamination, instructions on oral hygiene measures given to the patient and inclusion of the patient into a support therapy programme in which, during each recall, the state of the peri-implant tissues is evaluated and appropriate treatment protocols utilised. Given that modern dentistry offers many effective methods for dealing with peri-implant diseases, their early recognition is vital to stop the progression of the disease and prevent unnecessary implant loss.