Even though the effectiveness of psychotherapy is generally acknowledged, researchers are yet to agree on a plausible explanation for this effectiveness or on possible mechanisms of change that are activated by psychotherapy. To enhance developments in these areas some researchers have called for a focus on treatment principles rather than treatment techniques. In this respect, the technique of exposure is instructive. Despite its common use with anxiety disorders and the successful outcomes it produces, it has only recently been considered as a treatment for other disorders. By focussing on the underlying principles of exposure it is possible to consider exposure as a transdiagnostic component of successful psychotherapies. Understanding exposure from the perspective of Perceptual Control Theory (PCT) enables the identification of a functional, rather than a conceptual or statistical, mechanism of change. Functionally, exposure can be understood as an essential precursor to the internal reorganization that is necessary for the amelioration of psychological distress. PCT suggests a more considered and widespread use of exposure in psychotherapy as a way of improving both the efficiency and the effectiveness of the treatments offered. Despite the proliferation of psychotherapies in recent years there has not been a commensurate growth in our understanding of the effective ingredients of treatment. It seems unlikely that there are multitudes of different mechanisms and processes through which psychological distress is resolved and that each of these different psychotherapies utilises a distinct item from the collection. In fact, the spawning of hundreds of different psychotherapies is perhaps the most telling sign that there may still be much work to do before the important principles of treatment are described and fundamental mechanisms of change are identified. Could a technique as mundane as exposure hold the key to effective psychotherapy? For psychotherapy, is there just one road leading to Rome but a plethora of ways to travel that road?