  The composition of cometary ices provides key information on the thermal and
chemical properties of the outer parts of the protoplanetary disk where they
formed 4.6 Gy ago. This chapter reviews our knowledge of composition of
cometary comae based on remote spectroscopy and in-situ investigations
techniques. Cometary comae can be dominated by water vapour, CO or CO2. The
abundances of several dozen of molecules, with a growing number of complex
organics, have been measured in comets. Many species that are not directly
sublimating from the nucleus ices have also been observed and traced out into
the coma in order to determine their production mechanisms. Chemical diversity
in the comet population and compositional heterogeneity of the coma are
discussed. With the completion of the Rosetta mission, isotopic ratios, which
hold additional clues on the origin of cometary material, have been measured in
several species. Finally, important pending questions (e.g., the nitrogen
deficiency in comets) and the need for further work in certain critical areas
are discussed in order to answer questions and resolve discrepancies between
techniques.
