The technical requirements to demonstrate values of underbalanced drilling projects before taking an executive decision could be overwhelming and probably hurt the advancement of a great opportunity. This paper will discuss the recent research results on inflow performance of underbalanced wells with special focus on tools developed to distill the elements and other parameters contributing to the enhanced flow in UBD wells. It will address the inherent technical challenges and solutions in predicting the productivity performance of a low skin, high producer wells. The methodology and critical elements involved in the analytical and numerical solutions offered will be discussed. The range of validity of application of these methods will also be covered. Also discussed, will be the inflow characteristics of horizontal and extended-reach wells and why these categories of wells benefit from newly developed technology. Discussion will also outline the results of the formulation, to illustrate the reservoir-engineering framework necessary to capture the near wellbore effect. Finally, extension of mathematical equations previously prescribed for the response of vertical or horizontal wells, within the drainage volume of any reservoir geometry will be included.