Signal theory is the theory of the engineering discipline of signal processing. It provides mathematical approaches for use in a wide range of applications. Fields as far apart as statistical physics and op-amp electronics share some of this material.

High-level aspects include:

• Amplitude,
• phase,
• resonance,
• Q (Q for "quality", means the strength of a resonance),
• time constant (of a filter),
• great-signal and small-signal bandwidth,
• maximum output swing ,
• noise floor.