There are many individual pieces of equipment on a rotary drilling rig (Figure 1). These individual pieces of equipment can however be grouped together into six sub-systems. These systems are: the power system; the hoisting system; the circulating system; the rotary system; the well control system and the well monitoring system. Although the pieces of equipment associated with these systems will vary in design, these systems will be found on all drilling rigs. The equipment discussed below will be found on both land-based and offshore drilling rigs.