Abstract The conventional overbalanced drilling of production zones generally results in a negative effect on overall productivity and cost, caused primarily by formation damage. As a result, the oil industry is turning more toward methods such as underbalanced drilling (UBD) to increase productivity and reduce overall costs. UBD, particularly pressure-depleted, mature reservoirs, can minimise or prevent near-wellbore damage, often avoiding the need for stimulation. A clear and fundamental understanding of drilling technology is required to design and supervise drilling operations on an underbalanced well. The benefits of UBD technology, the use of CT (coiled tubing) in UBD, UBD fluid systems, control of the well while drilling are presented.