There are different phases of hair growth. The length of each one varies depending on the anatomical, hormonal changes, medical treatments taken and other factors. These factors also influence the number and frequency of sessions needed for permanent hair removal. Anagen. The hair is growing and is therefore visible. In this phase, the light is absorbed by the hair color converting it into heat energy and transmitting the heat to the follicle, destroying the follicle. Catagen. The hair is present, but it is separated from the matrix, so that the heat conduction to the follicle is not achieved. The treatment is not effective.Exogen. The hair is leaving the follicle.- Telogen. Is the resting stage, there is no hair. The session is not effective. Consequently several sessions are necessary to complete process of permanent hair removal: we must ensure that the session to act on the highest possible percentage of hairs in the anagen phase.