This paper briefly describes the fundamental principles of the instruments used for accurate measurement of hydrostatic pressure and in particular the use of piston gauges as primary pressure standards. Different methods for the calibration of secondary standards have been discussed and in particular, emphasis has been given to the calibration of secondary piston gauges against the primary standards by the cross-float method along with the evaluation of uncertainties attached to different correction factors associated with the measurement of pressure from these gauges. The importance of secondary pressure standards in the region 0.1 GPa to several GPa has also been defined.