This review covers the different techniques that are available for the calibration of vacuum gauges, dealing with both calibration by comparison with an absolute gauge and calibration on a primary standard which generates known low pressures. The various sources of systematic uncertainty in both the series expansion and the orifice flow methods are examined and quantified. The performances of some calibration systems are compared. Parts of this review also cover the possible use of fixed points in vacuum metrology, reference gauges and the intercomparison of standards between laboratories.