Sublimation of the volatile ices in the cometary nucleus when approaching the Sun leads to the formation of the extended cometary neutral gas coma. The ultimate goal of coma observations is to learn the composition of cometary nuclei and, therefore, of the probably least evolved bodies of our solar system. However, concluding from measurements of the coma to the nucleus is not straightforward. Abundance ratios and composition differ from the nucleus ices because of differentiation as a consequence of sublimation and chemical reactions during expansion in the coma. In addition, converting measured fluxes to production rates requires knowledge of the excitation process of the emissions recorded and of the gas expansion field.

Nevertheless, remote observations of the gaseous coma in the radio, optical, but also the infrared and ultraviolet range can provide significant information and constraints on nucleus composition. This is well demonstrated by the recent observations of comets Hyakutake and Hale-Bopp, resulting in the detection of many new species and abundance measurements of parent and daughter species over a wide range of heliocentric distances. In particular observations of the most volatile molecules, in comparison to water, give the most servere constraints on the physical structure of the nucleus ices. In addition, measurements of isotopic ratios allow to access quantities which are less subject of modification in the sublimation and expansion process.

The implications on nucleus composition derived from Earth-based observations of the gaseous coma will be critically discussed.