1. A computer network policy implementation system for causing network control operations to be performed in response to changes in state of the network comprising: a policy editor having user interface through which a user may cause a set of policy instructions to be stored to define the network policy, said stored set of policy instructions setting forth operations to be performed in response to changes in state of the network; network monitoring means for monitoring at least one of a predefined set of changes in the network and for providing system state signals indicative of said changes; action engine means for selectively causing at least one of a predefined set of network control operations to be performed; a policy interpreter, receptive of said stored set of policy instructions and communicating with said network monitoring means, for building at least one data structure to represent said stored set of policy instructions in relation to store said system state signals; said policy interpreter implementing a predefined language having at least a first mechanism to specify that action should be initiated in response to a state signal, and a second mechanism to specify that action should be terminated in response to a state signal whether or not the action has been initiated; said policy interpreter including an executor means for managing a plurality of threads each representing at least a portion of the policy instructions, the threads executing concurrently and interactively with one another; said executor means communicating with said action engine means to cause network control operations to be performed in response to changes in state of the network in accordance with said stored set of policy instructions.