This book is the result of the enthusiasm that, throughout his professional life, the author has shown for unravelling the mysteries whereby some patients, and in them some teeth, resist the ravages of time and periodontal disease, yet others do not. After analysing an enormous amount of data over 25 years, his conclusions contribute towards a greater precision in establishing the prognosis of patients' teeth, with a higher empirical reliability than was available before now.The text introduces the LTOP system for assigning comprehensive periodontal prognosis by integrating overall prognosis and individual tooth prognosis. This approach is based on data gathered during more than 25 years of research on periodontal prognosis and includes the following contributions: development of Perioproject, a prediction model of tooth loss due to periodontitis; development of the LTO index (Long-term outcome index) to identify, at baseline, the reduced percentage of patients with the highest risk of tooth loss, as well as identifying the two different profiles of patients (Type 1 and Type 2) associated with bruxism and at risk of losing more teeth; last but not least, development of the LTOP system to assign periodontal prognosis by integrating the LTO index and the estimated survival time assigned to periodontally involved teeth with Perioproject.