Theories established from engineering fundamentals have been of great value in supporting the design and execution of drilling operations in gas drilling where gas is used as a drilling fluid. This work presents an overview of new theories developed in recent years for special gas drilling operations including horizontal wells. These new theories are found in the areas of gas-mixture flow hydraulics in deviated and horizontal boreholes, hole cleaning of solids accumulation, hole cleaning of formation water, flow diverging for washout control, bit orifice optimization, and depression of formation water influx. This paper provides drilling engineers with updated mathematical models and methods for optimizing design to improve gas drilling performance.