Many bioengineering techniques to elucidate functions and characteristics of the skin have been developed. They measure such factors as the water content of the 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 images or vertical cross sections of the skin with subcellular resolution, have been developed by using the non-linear optics phenomenon. In this paper, I review the skin measurement technologies that have been used for cosmetics efficacy test, and also describe in vivo bio-microscopes, which include optical coherence tomography (OCT), the confocal laser scanning microscope (CLSM), the second harmonic generation (SHG) microscope, the multi photon microscope (MPM), and coherent anti-Stokes Raman scattering (CARS), which will be very helpful to examine the effects of cosmetics on the skin.