Ferroresonance is a non-linear resonance phenomenon that can affect power networks. The abnormal rates of harmonics and transient or steady state overvoltages and overcurrents that it causes are often dangerous for electrical equipment. Some unexplained breakdowns can be ascribed to this rare, non-linear phenomenon. The purpose of this "Cahier Technique" is to help readers understand ferroresonance. The methods described ensure credible prediction and evaluation of the risk of ferroresonance in existing and future installations. Practical solutions designed to avoid or eliminate ferroresonance are also given.