Increasing soil salinity is a serious threat to agricultural productions worldwide in the 21st century. Several essential Na+ transporters such as AtNHX1 and AtSOS1 function in Na+ tolerance under salinity stress in plants. Recently, evidence for a new primary salt tolerance mechanism has been reported, which is mediated by a class of HKT transporters both in dicots such as Arabidopsis and monocot crops such as rice and wheat. Here we present a review on vital physiological functions of HKT transporters including AtHKT1;1 and OsHKT1;5 in preventing shoot Na+ over-accumulation by mediating Na+ exclusion from xylem vessels in the presence of a large amount of Na+ thereby protecting leaves from salinity stress. Findings of the HKT2 transporter sub-family are also updated in this review. Subjects regarding function and regulation of HKT transporters, which need to be elucidated in future research, are discussed.