This paper aims to review recent results on the optical clearing of the naturally turbid biological tissues and blood using the optical immersion technique, which is well known in physical science and is applied for the reduction of light scattering and undesirable reflections in the optical system. Basic principles of the technique, its advantages, limitations and future are discussed. The refractive index matching concept for enhancement of in-depth light penetration into tissues and blood is presented on the basis of in vitro and in vivo studies using optical spectroscopy, polarization and coherence-domain techniques. The index matching of scatterers and ground matter by means of administration of clearing agents is under discussion. The optical properties of tissues with basic multiple scattering, which are transformed to a low scattering mode, are analysed. It is shown that light reflection, transmission, scattering and polarization can be effectively controlled. The possibilities of using the optical immersion method for diagnostic purposes based on contrasting of abnormalities, on in-depth profiling of tissue and blood and on monitoring of endogenous and exogenous matter diffusion within tissue are demonstrated.