A deep hole is defined as one that is a minimum of 10 times its diameter in depth. On the maximum side, a deep hole drilling is generally capable of drilling to a depth of 100 to 150 times its diameter. And hole deeper than 10 times the diameter should certainly be considered a deep hole, requiring a specialised drilling technique.

Deep hole drilling are characterized by high material removal rate plus high accuracy with regard to the straightness of the hole, surface finish and dimensional tolerances. The extreme conditions which apply when drilling deep holes place high demands on the tool, machine and associated equipment.