As in many fields today, a significant gap has developed between theory and practice in exhaust system design. The theoretician often does not appreciate the relevant practical concerns associated with the design process, while the designer often has difficulty in properly interpreting theoretical results for practical use. The goal of this paper is to present a number of theoretical results related to exhaust systems and discuss some of their practical implications for design. A brief review is included of exhaust system theory as well as experimental results obtained on actual units, The connection between theory and practice is then analyzed for reactive effects, resistive effects, and engine interactions. The emphasis throughout the paper is on the use of theory to guide practical design.