Cometary nuclei consist of ices intermixed with dust grains and are thought to be the least modified solar system bodies remaining from the time of planetary fo rmation. Their structure and composition yield insights into processes occurring during that time, and provide a link to processes currently taking place in reg ions of planetary system formation. However, a cometary nucleus can be analyzed only if a dedicated spacecraft mission would land on its surface. Ground-based a nd near-Earth space observations of comets deal with remote sensing of gas and d ust released from the nucleus into the coma. Spacecraft missions to comet Halley measured in situ the elemental composition of more than 5000 individual grains with mass in the range of 5x10^-1 - 5x10^-12 g. These direct measurements of the only proven sample of cometary solid material are of key importance for underst anding the composition of cometary bodies. The main results can be summarized as follows.

• Cometary particles are fine scale mixtures of carbonaceous and inorganic phase s, with the organics-to-minerals mass ratio between 0.5 and 1.
• The bulk abundances of the major rock-forming elements integrated over all spe ctra are solar (CI-chondritic) within a factor of <2.
• The volatile elements H, C, and N are enriched relative to their CI-chondritic abundances indicating that cometary solid material is more primitive than CI-chondrites.
• The partion of the volatile elements between gas and dust is 2:1 for H and O (as expected for a water-rich object), and 1:2 for C and N (i.e. these elements o ccur predominantly in the dust). The abundances of C and N in cometary dust are close to the abundances of these elements in the dust in dense molecular clouds.
• Minerals possibly occurring in the cometary dust include: Fe-poor and Ca-poor Mg-silicates, Fe sulfides (some Ni-rich), Fe oxide and Fe metal (Ni-poor).
• The presence of Mg carbonates and of layer silicates suggests the possibility of aqueous alteration of cometary dust.
• Carbonaceous and organic compounds of the dust include: elemental carbon grain s, aliphatic and aromatic hydrocarbons, and polymers of carbon suboxide and of c yanopolyynes. Cometary dust also appears to contain heteropolymers and/or variab le mixtures of various alcohols, aldehydes, ketones, acids and amino acids, and their salts.
• The observed compositional diversity of individual dust grains underscores the unequilibrated nature of cometary material and is consistent with the interstel lar dust model of comets.