  Tortuosity is one of the key parameters describing the geometry and transport
properties of porous media. It is defined either as an average elongation of
fluid paths or as a retardation factor that measures the resistance of a porous
medium to the flow. However, in contrast to a retardation factor, an average
fluid path elongation is difficult to compute numerically and, in general, is
not measurable directly in experiments. We review some recent achievements in
bridging the gap between the two formulations of tortuosity and discuss
possible method of numerical and an experimental measurements of the tortuosity
directly from the fluid velocity field.
