A formal model for representing control in production systems is defined.

The formalism allows control to be directly specified independently of the

conflict resolution scheme, and thus allows the issues of control and

nondeterminism to be treated separately. Unlike previous approaches, it

allows control to be examined within a uniform and consistent framework.

It is shown that the formalism provides a basis for implementing control

constructs which, unlike existing schemes, retain all the properties

desired of a knowledge based system --- modularity, flexibility,

extensibility and explanatory capacity. Most importantly, it is shown

that these properties are not a function of the lack of control

constrains, but of the type of information allowed to establish these

constraints.

Within the formalism it is also possible to provide a meaningful notion of

the power of control constructs. This enables the types of control

required in production systems to be examined and the capacity of various

schemes to meet these requirements to be determined.

Schemes for improving system efficiency and resolving nondeterminism are

examined, and devices for representing such meta-level knowledge are

described. In particular, the objectification of control information is

shown to provide a better paradigm for problem solving and for talking

about problem solving. It is also shown that the notion of control

provides a basis for a theory of transformation of production systems, and

that this provides a uniform and consistent approach to problems involving

subgoal protection.