A review of our reduced-dimensionality quantum dynamical treatments of the coupled low-frequency large-amplitude torsional vibrations of the water trimer is presented. It covers three-dimensional bound state calculations of the intermolecular torsional energy levels of (H 2 O) 3 , (D 2 O) 3 , and other isotopomers, and quantum (3+1)-dimensional torsional-stretching calculations of torsional-state dependent vibrationally averaged rotational constants of isotopically pure and mixed water trimers.