Peri-implant diseases represent serious complications after dental implant treatment, affecting both the surrounding soft and hard tissue. Regular, specific check-ups with the assessment and elimination of risk factors are effective precautions, and various conservative and surgical approaches are available for the treatment of peri-implant diseases. Early detection of clinical signs diseases and timely treatment are important for the success of dental implant treatment. Dentists must be familiar with the radiological and clinical characteristics of both conditions to make an accurate diagnosis and determine the appropriate treatment. The purpose of this chapter is to provide advice on the diagnosis and prevention of peri-implant disease for practitioners and to offer an overview of current knowledge on the treatment of peri-implant diseases.