  We discuss models that astrochemists have developed to study the chemical
composition of the interstellar medium. These models aim at computing the
evolution of the chemical composition of a mixture of gas and dust under as-
trophysical conditions. These conditions, as well as the geometry and the
physical dynamics, have to be adapted to the objects being studied because
different classes of objects have very different characteristics (temperatures,
densities, UV radia- tion fields, geometry, history etc); e.g., proto-planetary
disks do not have the same characteristics as protostellar envelopes. Chemical
models are being improved continually thanks to comparisons with observations
but also thanks to laboratory and theoretical work in which the individual
processes are studied.
