This chapter presents some very useful techniques for the cancellation of noise coherent with an available reference signal or when the noise can be modeled in a predictive way. Active noise control is mostly an academic curiosity at the time of the writing of the second edition, but its commercial limitations are primarily a cost and maintenance issue, not a feasibility concern. Contact noise is directly proportional to the dc current across any contact in the circuit, such as solder joints, switches, wire connectors, and internal component materials. Popcorn Noise is also known as burst noise, and gets its name from the sound heard when it is amplified and fed into a loudspeaker. Noise reduction is both straightforward and useful to a wide range of signal processing applications. In active noise control, one must take into account the possibility of feedback from the physical actuator back into the reference signal.