  Isotopic ratios provide a powerful tool for understanding the origins of
materials, including the volatile and refractory matter within solar system
bodies. Recent high sensitivity observations of molecular isotopologues, in
particular with ALMA, have brought us new information on isotopic ratios of
hydrogen, carbon, nitrogen and oxygen in star and planet forming regions as
well as the solar system objects. Solar system exploration missions, such as
Rosetta and Cassini, have given us further new insights. Meanwhile, the recent
development of sophisticated models for isotope chemistry including detailed
gas-phase and grain surface reaction network has made it possible to discuss
how isotope fractionation in star and planet forming regions is imprinted into
the icy mantles of dust grains, preserving a record of the initial isotopic
state of solar system materials. This chapter reviews recent progress in
observations of molecular isotopologues in extra-solar planet forming regions,
prestellar/protostellar cores and protoplanetary disks, as well as objects in
our solar system -- comets, meteorites, and planetary/satellite atmospheres --
and discusses their connection by means of isotope chemical models.
