An Effects Loop is a signal patch point usually found on guitar and bass amplifiers that allows external effects units to be placed between the amplifier's two main stages: the preamplifier (which shapes the tone and creates gain/distortion) and the power amplifier (which amplifies the signal to drive the speakers).

It typically consists of a Send output and a Return input. This positioning is critical for signal hygiene. Modulation and time-based effects like reverb and delay generally sound much clearer when placed in the loop. This is because they are applied to the already-shaped (and potentially distorted) tone. If these effects were placed in front of a high-gain amp, the subsequent distortion would compress and mangle the reverb tails or delay repeats. While similar in concept to a mixer's Insert Point, amplifier effects loops are specifically designed to interface with instrument-level or line-level effect pedals and rack units.