This research is concerned with the problem of characterizing the compensator sturcture associated with the linear time invariant multivariable servomechanism problem. A study of the compensator sturcture associate with both robust and non-robust solutions is made and necessary and sufficient conditions for a solution to exist are found. The robust servomechanism problem is solved for two cases:​ (i) when the system's parameters are only perturbed and (ii) when the system's feedback parameters are only perurbed. The special case of the above servomechanism problem, when the regulated variables are only fed back is then studied extensively. A frequency domain interpretation of the above results is also included. Systems with time delay are then investigated. In particular, the servomechanism problem is solved for a general class of time delay systems and a characterization of rubst controllers for such systems is made.