Nonsurgical periodontal therapy is directed toward removal of the microbial biofilm from the root surfaces of periodontally diseased teeth. Conceptually, the aim of this treatment approach is to eliminate both living bacteria in the microbial biofilm and calcified biofilm microorganisms, i.e. dental calculus,from the root surface and from the subgingival area without the surgical reflection of the soft tissues surrounding the teeth. From a practical point of view the result of this treatment is a more or less complete removal of the calcified accretions covering the root surfaces, a reduction of the number of biofilm microorganisms, and a disturbance of the ecology of the microbial biofilm. Mechanical nonsurgical periodontal therapy in this review includes all nonsurgical treatment modalities that are performed with the use of hand instruments, sonic or ultrasonic instruments, motordriven instruments, or any combination of these instruments.