**1. Introduction**

Lasers are becoming widely used in medicine and dentistry due to their beneficial effects such as: coagulation properties (less postoperative bleeding), less pain and edema. Lasers also allow good and rapid healing, a very low level of discomfort both during and after intervention and a rapid disappearance of symptoms.

**Four responses within tissues** are described when the laser beam hits the target tissue namely reflection, absorption, transmission and scattering. The main mechanisms of interaction between the lasers and biological tissues are: photothermic, photoacoustic and photochemical. The effect of lasers on the soft tissues is based on the transformation of light energy into heat. Operator-dependent factors affecting the effect of lasers are: power density, energy density, pulse repetition rate, pulse duration and the mode of energy transferred. Operator nondependent factors which affect laser treated areas are specific laser wavelengths and optical properties of the target tissues [1].

**Effects on the tissues** when lasers are applied include the increase in the temperature, coagulation, hemostasis, tissue sterilization, tissue welding, incision, excision, ablation and vaporization [1]. When laser energy is absorbed in the water of the hard tissues, a rapid volume expansion of the evaporating water occurs as a result of a substantial temperature elevation at the interaction site. Micro-explosions are produced causing hard tissue disintegration. If pulp temperatures are raised beyond 5 degrees, pulp damage is irreversible. If heat is intensive and lasts for an extended period of time the consistency of the intracellular ground substance may not be preserved [1].

© 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

**Erbium-yttrium-aluminum-garnet (Er: YAG) lasers** produce invisible infrared light at a wavelength of 2.940 nm which is ideal for absorption by hydroxylapatite and water [2]. Therefore, they can be used for treatment of both soft and hard tissues (unlike for example diode lasers). As the Er: YAG wavelength corresponds to the absorption coefficient of water, Er: YAG laser irradiation transforms water within tissue into steam leading to the development of micro-explosions [3].
