A tooth is an unusual mineralized structure, passing through the integument of human body, partly exposed to the external environment while partly within the connective tissue. Periodontal tissues thus possess specific anatomic structures to cope with disease invasions. A dental implant is a replacing material to restore the function and esthetics of a missing tooth. Whether the pen-implant tissue contains similar abilities as periodontal tissues, it remains further investigations in dentistry. Nowadays we realize when tissue adjacent to these structures was infected or ill, i.e., periodontal disease or pen- implant lesion, there are similarities noted. This paper discusses the classifications, bacterial etiology, diagnostic parameters and treatments of periodontal diseases and pen-implant lesions. By knowing periodontitis, it provides a precious experience for clinicians to diagnose and treat peri-implantitits.