AAA+ family proteolytic machines (ClpXP, ClpAP, ClpCP, HslUV, Lon, FtsH, PAN/20S, and the 26S proteasome) perform protein quality control and are used in regulatory circuits in all cells. These machines contain a compartmental protease, with active sites sequestered in an interior chamber, and a hexameric ring of AAA+ ATPases. Substrate proteins are tethered to the ring, either directly or via adaptor proteins. An unstructured region of the substrate is engaged in the axial pore of the AAA+ ring, and cycles of ATP binding/hydrolysis drive conformational changes that create pulses of pulling that denature the substrate and translocate the unfolded polypeptide through the pore and into the degradation chamber. Here, we review our current understanding of the molecular mechanisms of substrate recognition, adaptor function, and ATP-fueled unfolding and translocation. The unfolding activities of these and related AAA+ machines can also be used to disassemble or remodel macromolecular complexes and to resolubilize aggregates.