Microfluidics is a relatively new field of research that has grown significantly over the past few decades. It takes its name from fluidics, a discipline that appeared in the 1960s and was entirely dedicated to internal gas flows at a mini and micro scale. Paradoxically, microfluidics was initially focused mainly on liquid microflows, but applications of gaseous microflows today affect more and more numerous and varied sectors. Internal gas flows can be rarefied either by decreasing the pressure or by reducing the characteristic dimensions of the flow, as is the case inside microfluidic systems. The rarefied regimes observed present original particularities, such as the phenomenon of thermal transpiration which can be exploited to generate flows by means of a single temperature gradient. These phenomena are widely studied theoretically and simulated numerically using various models, but their experimental analysis is still poorly developed, suffering from difficulties linked to very high confinement. In this seminar, we present the advantages of molecular marking techniques and their ability to access the velocity, pressure and temperature fields within gas microflows. After a presentation of the method and the results obtained, the current limits and prospects of these techniques are then discussed.