Computer-based circuit simulation is often used as a tool by designers, test engineers, and others who want to analyze the operation of a design without examining the physical circuit. Simulation allows you to change quickly the parameters of many of the circuit elements to determine how they affect the circuit response. Often it is difficult or impossible to change these parameters in a physical circuit.

However, to be practical, a simulator must execute in a reasonable amount of time. The key to efficient execution is choosing the proper level of modeling abstraction for a given problem. To support a given modeling abstraction, the simulator must provide appropriate algorithms.

Historically, circuit simulators have supported either an analog simulation algorithm or a digital simulation algorithm. Ngspice inherits the XSPICE framework and supports both analog and digital algorithms and is a `mixed-mode' simulator.