The knowledge of how acoustic waves propagate from one acoustic medium to another, and what happens to the acoustic waves when they encounter obstacles to their propagation, is a prerequisite for any NVH engineer and researcher. Acoustic waves propagate unperturbed in a uniform homogenous medium. Acoustic medium changes whenever there is a change in bulk modulus or mass density or both of the medium. The change in acoustic medium offers resistance to wave propagation and this resistance is quantified in terms of the acoustic impedance of the medium. The wave as a result undergoes reflection, transmission, and absorption at the medium boundaries. The phenomena of reflection, absorption, and transmission has been described in detail in this chapter. Important relationships among reflection, absorption, and transmission coefficients have been derived, and the knowledge gained will form a solid foundation for noise control engineers and researchers.