The analogue of metabolic control theory is developed for the control of reactions catalyzed by single enzymes. The control exerted by any of the elemental transitions of enzyme catalytic cycles on reaction rate and on concentrations (probabilities) of enzyme states is quantified in line with the principle of detailed balance. For enzyme reactions with arbitrary kinetic schemes, e.g., with several enzyme cycles, reflecting coupling and slipping of reactions, it is derived what the various sums of the control coefficients are equal to (cycle summation theorems). Total control on flux, state probability and ratios of branch fluxes are 1, 0 and 0, respectively. The general connectivity theorems are derived which indicate how control is determined by the kinetics of the elemental steps. In addition, for enzymes catalyzing single (or completely coupled) processes the control coefficients are expressed in terms of actual and standard free energy differences across the steps. The prevalent qualitative contention that the step with the smallest forward rate constant, or with the largest free energy drop is the step limiting the performance of the enzyme is shown to fail. The new theory should allow subtle analysis of the control of an enzyme catalyzed reaction.