A model describing the fundamental working principle of the "ringcore fluxgate sensor" is derived. The model is solely based on geometrical and measurable magnetic properties of the sensor and from this a number of fluxgate phenomenon can be described and estimated. The sensitivity of ringcore fluxgate sensors is measured for a large variety of geometries and is for all measurements found to fall between two limits obtained by the fluxgate model. The model is used to explain the zero field odd harmonic output of the fluxgate sensor, called the "feedthrough". By assuming a non ideal sensor with spatially distributed magnetization, the model predicts feedthrough signals which exactly reflects the measured signals. The non-linearities in a feedback compensated ringcore fluxgate sensors, called the "transverse field effect", can also be explained by the model. Measurements on stress annealed amorphous ringcore fluxgate sensors show that using the "short-circuited detector coil" and the "all even harmonic detection" principles, the transverse field effect is in the order of 1 nT compared to 25 nT for traditional ringcore fluxgate magnetometers.The calibration of the triaxial fluxgate sensor is eased by the use of a new scalar calibration method. It is demonstrated, that a calibration with residuals below 0.2 nTrms can be obtained using only the magnetic field of the earth. This result is comparable with the best results ever produced in a coil facility. By this method it is also possible to partially compensate for transverse field effects. The demagnetization factor of the ringcore is a key parameter in this and other models of the ringcore fluxgate sensors. An approximation to the demagnetization factor, based on the field inside a hollow cylinder, is given, which compared to measurements typically gives a value within 5%. The stress annealing procedure is described by which an amorphous VITROVAC 6025 ribbon can be produced with no magnetic hysteresis and no magnetostriction. This ribbon is found to be very suitable for use as core material in fluxgate sensors.