Laser-Induced Damage Threshold, or LIDT is the highest fluence or power density of laser radiation incident upon the optical component before any unacceptable damage occurs. This article offers a basic tutorial and explanation of laser damage threshold, including the definition of LIDT The test method of LIDT, laser-induced damage threshold for CW laser and pulsed laser, the calculation formula, the laser damage mechanisms, how to scale LIDT, the causes of laser damage, and high power laser optics/coatings.