Light can destroy hair follicles via 3 mechanisms: thermal (due to local heating), mechanical (via shock waves or violent cavitation), and photochemical (through generation of toxic mediators like singlet oxygen or free radicals).

Laser hair removal is thought to work through selective damage to the hair follicles and this mechanism is based on the principles of selective photothermolysis. Melanin acts as the chromophore for targeting hair follicles, and the lasers or light sources that are used for hair removal lie within the optical window of the electromagnetic spectrum where absorption by melanin and deep penetration into the dermis are combined. Within the 600–110 nm region, deep and selective heating of the hair shaft, hair follicle epithelium, and hair matrix is possible while selective cooling of the epidermis minimizes epidermal injury and damage to epidermal melanin.
Appropriate selection of wavelength, pulse duration, and fluence are important in optimizing the hair removal while minimizing any potential side effects.A laser pulse that is too long can cause heat damage to the surrounding tissues, resulting in permanent scarring.
The normal mode ruby laser (NMRL) is one of the lasers that have been used as a depilatory. This laser only penetrates approximately 2 mm and the bulge region of the hair follicle lies deeper than 2 mm, therefore, it may be suggested that the lack of penetration to this area contributes to the poor depilatory results of this laser. An energy loss of up to 50% may occur as the laser penetrates through the first 1 mm of skin, and Topping et al. postulate that approximately 50% of the incident fluence reaches both the bulge region and the hair bulb.
When choosing treatment parameters, several factors must be considered. The hair cycle is important as the frequency of treatment must be timed to treat the hair in the anagen cycle when hair is more heavily melanized, however, as discussed below, there is controversy as to whether or not the hair cycle phase truly impacts laser hair removal effectiveness. Additionally, the hair diameter affects the optimal thermal relaxation time and pulse width. In order to obtain spatial confinement of thermal damage, the pulse duration must be shorter than or equal to the thermal relaxation time of the hair follicle; a thicker hair follicle will have a longer thermal relaxation time than a thinner hair. Thermal relaxation times of human terminal hair follicles are estimated to vary between 10–50 msec. Q-switched lasers operate within the nanosecond domain; therefore, they have a very small spatial scale of thermal confinement. Because of this property, these lasers damage individual pigmented cells within the hair follicles and confine heat. This property prevents any effective hair loss by Q-switched lasers. The depth of hair root (which impacts on the chosen wavelength, spot size, and energy) as well as the color of hair (which is directly related to the amount of melanin present) will also help in determining the most effective treatment parameters.

Prior to treatment, the possibility of adverse effects must be discussed with patients. Adverse events include, but are not limited to: hyperpigmentation, hypopigmentation, erythema, edema, scarring, pain, and blistering.

Beyond understanding the basic mechanics of laser hair removal, physicians seek to determine what the recommendations are regarding the ideal interval between sessions, what is the ideal number of sessions, should patients pluck, wax, or shave prior to undergoing laser hair removal, and what are the recommendations regarding sun exposure before and after treatment. Additionally, there are areas that should not be treated with laser hair removal. These guidelines are important in order to optimize treatment results and patient satisfaction while minimizing potential side effects.