Based on virtual equivalent system (VES) theory, this paper presents the viewpoint that the structure information of plant to be controlled and the convergence of parameter estimates are both unnecessary for the stability and convergence of various types of self-tuning control (STC) systems. A general self-tuning control system comprised of arbitrary control strategy, arbitrary parameter estimation algorithm, and arbitrary plant (deterministic or stochastic, minimum or non-minimum phase), is stable and convergent if only the control strategy stabilizes the parameter-known model; the parameter estimates are bounded and slow time varying with relative infinitesimal estimation error.