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Impedance mismatch refers to the situation in an electrical, electronic, or radio‑frequency system when the characteristic impedance of a source or load does not equal the impedance of the transmission line or circuit element to which it is connected. This mismatch causes a portion of the signal to be reflected back toward the source instead of being fully transmitted to the load. The degree of reflection is quantified by the reflection coefficient, which is calculated from the difference between the impedances.

In practice, impedance mismatch is a critical concern in a wide variety of applications. In audio systems, it can lead to distortion and reduced speaker performance. In telecommunications, mismatches can degrade signal strength and bandwidth, leading to data errors. In microwave and millimeter‑wave engineering, power loss due to reflections may cause heating and inefficiencies. Even in mechanical or optical analogues, impedance mismatch principles are used to describe energy transfer inefficiencies.

The phenomenon was first thoroughly formalized in the early twentieth century by Robert W. Woodward and James H. Ellis, who introduced the transmission line theory for high‑frequency circuits. Over time, engineers have developed a number of techniques to mitigate mismatches, including the use of tuned impedance matching networks, adaptive matching circuits, and broadband matching components such as transformers, stubs, and attenuators. In some cases, designers accept the mismatch and employ equalization or feedback mechanisms to compensate for the resulting distortions.

Overall, managing impedance mismatch is essential for optimizing power delivery, minimizing signal distortion, and ensuring reliable operation across many disciplines of engineering and physics.