Many bioengineering techniques to elucidate functions and characteristics of the skin have been developed until now. They measure the water content of stratum corneum, transepidermal water loss (TEWL), skin surface contour, mechanical properties, blood flow, sebum secretion rate, wrinkles, skin pigmentation and so on. These instrumental measurements are applied to validation of the efficacy of the cosmetics. Recently, a few in vivo bio-microscopes which non-invasively visualize horizontal image or vertical cross section of the skin with subcellular resolution, have been developed by using non-linear optics phenomenon. In this paper, I describe how skin measurement technologies have been changed with time and applied to cosmetic science, and also review in vivo bio-microscopes, they are optical coherence tomography (OCT), confocal laser scanning microscope (CLSM), second harmonic generation (SHG) microscope, multi photon microscope (MPM), and coherent anti-Stokes Raman scattering (CARS), which will be very helpful to examine the effects of the cosmetics on the skin.