Problems that may accompany downscaling in internal flows are discussed. Non-dimensional similarity parameters (like Re, M, etc.) depend on parameters that scale down fully (e.g., length), or that are specified by design (velocities, cascade geometry), or that are scale invariant (like thermophysical and transport properties of fluids).This fact has to be taken into acount in the similarity considerations. With decreasing Reynolds number the surface forces become of growing importance, however, if not going down directly to microflows the main problems are connected with the three-dimensional flow structure affecting the skin friction , and with the fact that some of the flow phenomena (e.g. vortical structures) cannot fully develop . Problems that may may downscaling in internal flows are discussed. Non-dimensional similarity parameters (like Re, M, etc.) depend on parameters that scale down fully (eg, length), or that are specified by design (velocities, cascade geometry) , or that are scale invariant (like thermophysical and transport properties of fluids) .This fact has to be taken into acount in the similarity considerations. With decreasing Reynolds number the surface forces become of importance importance, however, if not going down directly to microflows the main problems are connected with the three-dimensional flow structure affecting the skin friction, and with the fact that some of the flow phenomena (eg vortical structures) can not fully develop.