Chapter 7 deals with 3-D laminar boundary-layer instabilities and their control. It covers the full range of Mach numbers from incompressible to hypersonic. A practical example of a 3-D boundary layer is the flow over a swept wing, which is susceptible to four types of instabilities that can lead to turbulence onset. Of these, cross-flow instability is the most dominant and therefore the most studied 3-D boundary-layer instability mechanism. A fundamental understanding of the instability has led to methods of control that have been successfully demonstrated at incompressible to hypersonic Mach numbers. These and other methods of control are presented.