  The current research in graphene spintronics strives for achieving a long
spin lifetime, and efficient spin injection and detection in graphene. In this
article, we review how hexagonal boron nitride (hBN) has evolved as a crucial
substrate, as an encapsulation layer, and as a tunnel barrier for manipulation
and control of spin lifetimes and spin injection/detection polarizations in
graphene spin valve devices. First, we give an overview of the challenges due
to conventional SiO$_2$ substrate for spin transport in graphene followed by
the progress made in hBN based graphene heterostructures. Then we discuss in
detail the shortcomings and developments in using conventional oxide tunnel
barriers for spin injection into graphene followed by introducing the recent
advancements in using the crystalline single/bi/tri-layer hBN tunnel barriers
for an improved spin injection and detection which also can facilitate
two-terminal spin valve and Hanle measurements, at room temperature, and are of
technological importance. A special case of bias induced spin polarization of
contacts with exfoliated and chemical vapour deposition (CVD) grown hBN tunnel
barriers is also discussed. Further, we give our perspectives on utilizing
graphene-hBN heterostructures for future developments in graphene spintronics.
