The focus of this book is about time-domain numerical solution of hybrid electrical systems, in terms of electromagnetic fields, lumped current and voltage, or their combinations. An electromagnetic system can be considered either as distributed or lumped, sometimes as their combination. A distributed system is a system that demonstrates field behaviors, like propagation and interaction, and is described by field equations. A lumped circuit, on the other hand, can be simplified as a system that distance need not to be included in calculation of the field distribution. For a pure distributed system, field solution by a field solving technique is desired; for a lumped-only system, there are many mature methods to solve different states of the circuit. When an electronic system has both a distributed component and lumped component, the system is referred to as a hybrid system, and this will be the definition for hybrid system in this book.