This paper reviews the principles and the types of diagnostic tests for periodontal disease. The relevance of the tests, such as sensitivity and specificity, and the consequences after the tests are discussed. Several clinical and laboratory tests such as DNA probes; test for the presence of aspartate aminotransferase or collagenase in gingival crevicular fluid (GCF); and the genetic test for the patients having higher risk to the disease are presented. These tests can serve as the adjunctive tools for traditional periodontal examinations. Understanding and properly utilizing these diagnostic tests would enhance treatment and provide better prognosis for patients.