Molecular absorption lines at high redshift

The four known molecular absorption line systems at redshifts z$=$0.25--0.89 are discussed. Two of the systems originate in the galaxy hosting the `background' AGN, while in two cases the absorption occurs in truely intervening galaxies. Indirect evidence for the existence of very small scale structure in the molecular ISM is presented. A new absorption line at z$=$0.89 towards PKS1830--211 opens up interesting possibilities to accurately derive the mass of the lensing galaxy.