  Except for ortho- and para-H2, very little is known about nuclear spin
isomers (or spin modifications) of molecules. The main reason is the lack of
practical enrichment techniques. Recently a few enrichment methods were
developed, which opened up new possibilities in the field. These methods are
briefly reviewed. Substantial progress in the field has been made by the
introduction of Light-Induced Drift as a gas-phase separation tool. This is
illustrated by extensive data on CH3F, which reveal that the gas-phase
ortho-para conversion is governed by intramolecular mixing of the nuclear spin
states. The role of ``direct'' ortho-para transitions is shown to be small.
Various aspects of the conversion were investigated in detail: pressure and
collision partner dependence, isotope effect, temperature dependence. The most
decisive information on the spin conversion mechanism is derived from the
observation of level-crossing resonances in an electric field and the Quantum
Zeno effect induced by collisions.
