Many complex chemical processes having a large number of process variables and poorly understood models can be controlled reasonably well by controlling only a small subset of process variables using an equally small number of manipulated variables. This is the central premise of this article and is referred to as partial control. Knowingly or unknowingly, this idea has been and continues to be applied to successfully control numerous complex industrial processes. Despite its widespread use, partial control has never been explicitly formulated. The partial control problem is defined. A number of terms is introduced such as process variable dominance, modelable responses, practical degrees of freedom, and sufficiency of partial control. The new framework allows incorporation of both engineering-based decisions and more rigorous theoretical tools to achieve the goals of partial control. A number of practical examples illustrate the applicability of these concepts.