light quasi-elastically scatters off molecules and the velocity of the molecules convey a Doppler shift to the frequency of the scattered light. In molecular tagging techniques, like in PIV, velocimetry is based on visualizing the tracer displacements. Schemes MTV techniques have proven to allow measurements of velocities in inhospitable environments, like jet engines, flames, high-pressure vessels, where it is difficult for techniques like Pitot, hot-wire velocimetry and PIV to work. The field of MTV is fairly young; the first demonstration of implementation emerged within the 1980s and the number of schemes developed and investigated for use in air is still fairly small. These schemes differ in the molecule that is created, whether seeding the flow with foreign molecules is necessary and what wavelength of light is being used. In gases The most thorough fluid mechanics studies in gas have been performed using the RELIEF scheme and the APART scheme. Both techniques can be used in ambient air without the need for additional seeding. In RELIEF, excited oxygen is used as a tracer. The method takes advantage of quantum mechanical properties that prohibit relaxation of the molecule so that the excited oxygen has a relatively long lifetime. APART is based on the "photosynthesis" of nitric oxide. Since NO is a stable molecule, patterns written with it can, in principle, be followed almost indefinitely. Another well-developed and widely documented technique that yields extremely high accuracy is hydroxyl tagging velocimetry (HTV). It is based on photo-dissociation of water vapor followed by visualization of the resulting OH radical using LIF. HTV has been successfully demonstrated in many test conditions ranging from room air temperature flows to Mach 2 flows within a cavity. In liquids In liquids, three MTV approaches have been classified: MTV by direct phosphorescence (using a phosphorescent dye), absorbance (using a photochromic dye), and photoproduct fluorescence (typically using a caged dye). MTV based on direct phosphorescence is the easiest technique to implement because a single laser is needed to produce a luminescent excited molecular state. The phosphorescence signal is generally weaker and harder to detect than fluorescence. The second technique called MTV by absorbance relies on the reversible alteration of the fluorescence properties of a photochromic dye. The scheme showed good results in alcohol and oils, but not in water in which typical dyes are not soluble. The third variant of MTV was first deployed in liquids in 1995 under the name