A survey of existing instrumentation and developments is presented emphasizing instrumentation for in-flight calibration of vector magnetometers on magnetic mapping missions. Proton free or forced precession magnetometers are at the focus as calibration references, because the proton gyromagnetic ratio is a basic atomic constant for the SI units of magnetic and electric current. The classical proton free precession, the Overhauser forced oscillation and a new field cycling Overhauser are presented. Alkali metal vapor magnetometers, although not absolute in the same sense as the classical proton magnetometer, offer stability and resolution well suited for the calibration purposes. Recent developments are discussed. The metastable Helium magnetometer also offers quasi-absolute scalar measurements, and the use of semiconductor tuned lasers replacing an RF-excited Helium lamp holds great promise for improved accuracy and reduced power consumption.