This Chapter describes the significance of intruding sound in any space—that is, sound entering the space from outside. The level of audibility of intruding sound depends on masking by background sound, and requirements for audibility will depend on the nature of the space as well as the level of background sound. The chapter describes how to determine the level of intruding sound from other spaces and from the external environment; how to specify requirements for partitions to ensure that the level is appropriate; and common issues that limit achievement of good sound isolation. Associated spreadsheets provide ways to automate the calculations.