The characterisation of the structure and composition of interstellar grains is a fundamental issue because it is necessary to be able to link spectral variations, particularly in the infrared, to the evolution of the physical conditions of the environments. Moreover, this characterisation will make it possible to obtain a precise description of these grains in order to assess their contribution to the chemistry. Analogues of interstellar grains (PAHs, soot) are synthesised in the laboratory in order to study: their mode of formation and growth, their structure and organisation, their behaviour under irradiation and their spectroscopic characteristics.