Cometary water, the main constituent of nucleus ices, is difficult to observe diretly. Breakthroughs were made recently using new ground based infrared instruments and space techniques (ISO and SWAS) on comets C/1996 B2 (Hyakutake), C/1995 O1 (Hale-Bopp), 103P/Hartley 2, and C/1999 H1 (Lee): observations of water hot bands, of water rotational lines, and measurements of the [D]/[H] ratio and ortho-to-para ratio of water. We discuss the laboratory data necessary to model the water spectrum and to interpret these observations.