  This lecture aims to present an overview of the properties of Hall effect
devices. Descriptions of the Hall phenomenon, a review of the Hall effect
device characteristics and of the various types of probes are presented.
Particular attention is paid to the recent development of three-axis sensors
and the related techniques to cancel the offsets and the planar Hall effect.
The lecture introduces the delicate problem of the calibration of a
three-dimensional sensor and ends with a section devoted to magnetic
measurements in conventional beam line magnets and undulators.
