Propulsion system instability, which may be caused by distorted inlet flow, is a recurring problem which must be solved in each new aircraft development program. Trends in engine and airframe design that keep distortion sensitivity as a continuing problem are discussed. Sources of inlet flow distortion are catalogued; this information is used to assess the potential difficulties in development of a variety of aircraft types, e.g., fighter, attack, and transport aircraft. Methods for describing distortion both experimentally and conceptually are introduced. Sufficient background is stated to provide a perspective of the lecture series.