The drill string used in rotary drilling comprises many items, but the most important component in it is the drill bit. The drill bit disintegrates the formation under its teeth or cutting edges, and enables progress of the blasthole in the intended direction. In this process the bit undergoes very heavy wear and the penetration rate eventually drops down below an acceptable level. Thereafter the bit is required to be replaced. As a drill bit is an expensive and critical item, it is thoroughly dealt with in this separate chapter. Other accessories used in a rotary drill string are elaborated in chapter 6.