The design of controllers for multivariable systems, in particular large-scale systems, requires knowledge and exploitation of the structural properties of the systems. The ability to achieve certain structural properties, e.g. regulation, tracking, noninteraction, disturbance, rejection etc., can be determined on the basis of a small number of basic system concepts. In this paper, structural decomposition of a multivariable system is studied using the concept of a controllability subspace, and a review of recent work to investigate this concept is given.