Reservoir screening studies were carried out on all of ADCO’s reservoirs to determine their applicability for Underbalanced Drilling (UBD). These studies resulted in a prioritization of potential UBD candidates. The primary business benefits of UBD were determined as follows: –reduction of formation damage ○Productivity Improvement Factors (PIFs) much greater than 1 were determined in many cases;○potential elimination of complicated (and expensive) stimulation jobs.–dynamic reservoir characterization ○improved understanding of geological architecture/flow units and hence the potential for more accurate and reliable reservoir models;○improved completion designs. Some of the secondary benefits of UBD that were identified beforehand included Rate of Penetration (ROP) improvements while drilling and improved rig performance. In one field, it was demonstrated that the PIFs per well resulting from UBD would enable fewer wells to be used to develop the field by optimization of the horizontal well length and hence a significant reduction in capital expenditure. From these studies, two fields were selected to carry out initial trials. This paper describes the lessons learnt from the planning, design and subsequent implementation of UBD in the first well in ADCO, which involved a truly integrated and multi-disciplined team effort. An initial assessment of the business benefits obtained from the trial compared to the results of the initial screening studies is also provided.