  In dense starless and protostellar cores, the relative abundance of
deuterated species to their non-deuterated counterparts can become orders of
magnitude greater than in the local interstellar medium. This enhancement
proceeds through multiple pathways in the gas phase and on dust grains, where
the chemistry is strongly dependent on the physical conditions. In this
Chapter, we discuss how sensitive, high resolution observations with the ngVLA
of emission from deuterated molecules will trace both the dense gas structure
and kinematics on the compact physical scales required to track the
gravitational collapse of star-forming cores and the subsequent formation of
young protostars and circumstellar accretion regions. Simultaneously, such
observations will play a critical role in tracing the chemical history
throughout the various phases of star and planet formation. Many low-J
transitions of key deuterated species, along with their undeuterated
counterparts, lie within the 60-110 GHz frequency window, the lower end of
which is largely unavailable with current facilities and instrumentation. The
combination of sensitivity and angular resolution provided only by the ngVLA
will enable unparalleled detailed studies of the physics and chemistry of the
earliest stages of star formation.
