  The dynamics of a barred galaxy depends on the pattern speed of its bar. The
only direct method for measuring the pattern speed of a bar is the
Tremaine-Weinberg technique. This method relies on the analysis of the
distribution and dynamics of the stellar component. It is best suited to
gas-poor galaxies and therefore it has been restricted to early-type barred
galaxies. On the other hand, a variety of indirect methods, which are based on
the analysis of the distribution and dynamics of the gaseous component, has
been used to measure the bar pattern speed in late-type barred galaxies. The
complete sample of galaxies for which the bar pattern speed has been directly
measured with the Tremaine-Weinberg method is given. Nearly all the measured
bars are as rapidly rotating as they can be. By comparing this result with
recent high-resolution N-body simulations of bars in cosmologically-motivated
dark matter halos, it is possible to conclude that these bars are not located
inside centrally-concentrated halos.
