. Proof is a programming activity. Consequently programming environments which support proof in the large are required. We describe an environment which supports one area of proof-in-the-large: that of theory management. We present the notion of virtual theories. They give the illusion of multiple active theories allowing the user to switch between different theories at will, proving theorems and making definitions in each. The system ensures that proofs only use resources that are available in the environment of the current virtual theory. The code has been implemented on top of the HOL90 system. A side effect is that a version of autoloading is obtained for HOL90. A more radical feature that is obtained is the autoloading of tools. The system has been tested on part of a real hardware verification proof. Who controls the past controls the future, Who controls the present controls the past. George Orwell, Nineteen Eighty-Four 1 Introduction Interactive, machine-checked proof is ess...