Images of molecular gas in spiral galaxies show two types of bar structure: large-scale bars, which are correlated with the presence of stellar bars, and generally follow the spiral structure, and nuclear “bars” of a few hundred parsecs in size, which show no direct correlation with the presence of a stellar bar. The dynamical origin of nuclear molecular bars is unclear, although they are consistent with radial inflow of gas into the nucleus, and subsequent evolution of the gas structure.