A general scheme for time-delay control systems is presented so that with particular choice of system elements and loop gain many of the existing time-delay systems can be recovered. The effects of imperfections in the system model for time-delay systems are described and analysed. Sensitivity expressions are obtained and their relationship with the structure of time-delay systems are explored. Sensitivity Analysis for a time-delay system is extended to Adaptive Control of time-delay systems. An adaptive control system for Smith's Predictor and an LOP Optimal Control scheme are investigated analytically and by simulation. Delay-free optimal control methods are extended to systems with delay for LQP and parametric optimisation. For parametric optimal control, we explore, in particular, cases where deliberate mismatch leads to improvement, and the improvement of delay-free systems by addition of delay is also investigated. Various cases of LQP are discussed and the sub-optimality of published schemes, induced by mismatch is exposed. The method is also extended to time-varying systems with time-delay.