A control of a multi-variable process throughout a plurality of successive, distinct operational phases of the process involves accumulation of sets of values of the process and quality variables (a-j) from previous multiple operations of the process through all its phases, as respective datasets of a historical record. Each dataset includes an identifier (p) of the phase to which it relates. The current values of the applicable process variables (Qa-Qh) during each phase and the phase identifier (p) are indicated on respective axes (Xp,Xa-Xh) of a multi-dimensional display in relation to an operational envelope which defines bounds (UL,LL) for the values of the individual process and quality variables (a-j) relevant to that phase and which is derived from the datasets of the historical record identified with that phase. Changes in the current process variables (Qa-Qh) result in changes of the bounds (UL,LL), and alarm is given when a current value departs from within them. Either manually or automatically correction of an alarm condition is made in accordance with calculation of required change of value of one or more manipulatable process variables (a-c).