1. Introduction -- 2. Semiconductor physics : a summary -- 3. Galvanomagnetic effects -- 4. Hall plates and magnetoresistors : basic properties -- 5. Hall magnetic sensors -- 6. Hall devices as means for characterizing semiconductors -- 7. Magnetic-sensitive field-effect and bipolar devices -- 8. Comparing Hall sensors with other galvanomagnetic sensors -- App. A. International system of units -- App. B. Physical constants -- App. C. Properties of some semiconductors -- App. D. Magnetic quantities, units, relations and values -- App. E. Mathematical relations