This paper presents a simplified summary, expository in nature, of the theory of airborne sound insulation of walls. The normal incidence mass law for both single and double walls is derived and suitably approximated to give easily usable forms. The frequency dependence of transmission loss is explained and approximate formulae for calculation of the resonance frequency of a double wall deduced. The idea of wave coincidence, shear and bending effects, and flanking transmission in composite partitions, is introduced arid their effects illustrated. Finally, the physical principles of the reverberant room test for sound absorption arid sound insulation are shown and their meaning explained.