This paper describes the planning issues that will need to be addressed when planning underbalanced thru tubing drilling operations. Planning a conventional underbalanced well can be complicated at the best of times. Planning a thru tubing underbalanced operation is even more complicated. For a successful thru tubing operation, a significant amount of planning and experience is needed. Very often we see that the service providers for coil or UBD services get asked to help in the planning aspects of a well. Although their experience in executing operations associated with their equipment is significant, planning a thru tubing (TTD) well with the objectives of an operator in mind is not their daily business. We have seen a number of operational failures either as a result of poor planning or as a lack of experience. Many of these failures are being blamed on the equipment or on the technology as many operators are unaware of the current limits of TTD operations. This is partly due to the fact that many thru tubing drilled wells are experimental technology projects with operators testing if thru tubing operations can be executed on their own wells. It has been proven in Alaska and Oman that a sustained campaign of thru tubing underbalanced drilling can lead to significant cost savings and operational improvements. Yet in many area’s of the world, both underbalanced drilling technology and thru tubing drilling technology is not implemented due to uncertainties on the successful outcome of a TTD project. Experience in the planning and operational aspects of a TTD-UBD well is extremely important to achieve a successful outcome.