Data collected during XIX and XX centuries indicate a decrease in the strengh of Earth’s Magnetic Field (EMF). This phenomenon are bound to the origin of the Magnetic Field. From 1600 until today, several models have been presented trying to determine the origin of EMF. At this time, none is able to explain their characteristics. Here we present two of these models: The Self-Sustainting Dynamo Model, explains that the flow of charged particles in Earth’s outer core originates and reinforces the EMF. The movement of this flow is quite complex and chaotic. That’s why, from time to time, the polarity of EMF shifts (reversals). During a Reversal, the strength of the Field will decrease for long time then, it will shift its polarity, and it will increase again. The Decay Model has had a recently new impulse. According to this model, the electric currents in Earth’s outer core originated our first magnetic field, wich one have been decaying from then on, to an exponential rate. None of the presented models is free of difficulties, and the controversy among different models continuous, and it takes us to explore new possibilities.