Laser conditioning is a phenomenon which has been much explore during the last 15 years. Many optical components for high power laser have shown enhancement of their laser- induced threshold after under-threshold pre-irradiation. The improvement brought by laser conditioning is though to be necessary in the building of new NIF and LMF lasers, especially for dielectric multilayers and KDP crystals. However, we lack a complete physical explanation for the observations of conditioning. In particular, we do not know the effect of most physical parameters like pulselength. wavelength, repetition frequency of the laser shot. In this tentative review, we first gather results relevant to the general problem of laser conditioning, especially on dielectric multilayers irradiated with nanosecond pulselength. We find necessary to come back to the basic notion of damage and threshold. As a definition basis, we establish that laser conditioning is a damage with minor optical consequence and no continuing damaging with successive testing. This way of presenting the phenomenon contains a non-threshold description of optical damage. We finally explore some specific questions, that are technologically important for LMJ and NIF construction. Particularly, is laser conditioning necessary once we admit the occurrence of small damages, that is with a functional definition of damage threshold.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.