Fiber optics are used in medicine in power transmission for therapeutic use as well as in signal transmission for diagnostics. Fibers are the key component of the applicators which find a wide and varied use. Applicators with optical fibers are part of the standard equipment used with the various continuous and pulsed medical laser systems. Recently, optical fiber applicators for the Er:YAG, CO, and CO2 laser have become available. A combination of therapeutic and diagnostic fibers promises, for example in lithotripsy and angioplasty, not only to improve the safety level, but also to ensure the accuracy of the dosage and the precision of the laser energy applied. For the design of commercial fiber optic components a practical definition of the engineering damage threshold including a reasonable safety factor is given. With respect to this engineering damage threshold an overview of existing fiber technology with respect to the fields of application is presented and two examples of feedback systems to increase safety are discussed.