The specialty of public health medicine, it seems, is forever at a crossroads. So it was in 1974 at the time of the first NHS reorganisation, which established the specialty firmly within the health service. And so it was when Acheson reported on it in 1988 and reaffirmed its central purposes as 'the science and art of preventing disease, prolonging life and promoting health'.

But as another decade, and, indeed, the 20th century, closes, public health medicine once again finds itself at the crossroads. Only this time, far from its role being in doubt, public health medicine is facing a profusion of roles - more than it can reasonably, or sensibly, handle alone.

The dilemma has arisen as a direct result of the government's modernisation agenda for the NHS. The notion of clinical governance is a significant element and one which is proving more resource-intensive than envisaged. But at the same time as this particular agenda is growing, the government is also putting at the centre of its broader health policy a commitment to tackling inequalities in order to narrow the widening health gap between rich and poor.

The public's health is high on the policy agenda and lies at the heart of the government's determination to provide joined-up policy-making for joined-up problems. Some, like Acheson in his review of the evidence on health inequalities, have sought ingeniously to integrate the two agendas, with clinical governance being extended to embrace issues of access to services and equality of treatment. But most of those struggling to implement the government's ambitious reforms and seize the opportunities they provide are daunted by the range of the public health agenda that beckons.

Clinical governance creates a dilemma for public health medicine. Many of the components which fall within this umbrella concept, such as audit, guidelines and evidence-based medicine, have been around for some years and form a core part of the practice of public health medicine.

What is distinctive is that they are now being brought together to form a critical mass which is intended fundamentally to modify the culture of medicine, making it more transparent and accountable. Perhaps, more importantly, it brings management and medicine into a new relationship, and one where potentially - and certainly in theory - managers will exert far greater influence over what clinicians do.

Most lay managers, however, possess neither the expertise nor the confidence to use their newly acquired power. In these circumstances, they will probably turn to their public health colleagues since clinical governance demands sound clinical knowledge.

While welcoming their pivotal position at the centre of the attempt to grapple with long-standing issues concerning the effectiveness of clinical practice and the more traditional core business of public health, many frontline practitioners are worried about the ability of the specialty of public health medicine to deliver. This might matter less if public health specialists did not feel themselves to be on trial. If they fail to deliver they stand to lose credibility and give credence to their many critics, who assert that public health medicine cannot continue to exert a monopoly on all aspects of the public's health or pretend to be all things to all people.

It is now recognised that securing the public's health demands an array of skills, most of which are neither directly clinical nor to be found solely, or even primarily, in the NHS. There is no reason why a director of public health should have to be clinically qualified.

It is a protectionist fallacy to argue, as some public health specialists do, that only those with a medical training can command the respect of those with whom they must do business. Such assertions cut no ice in local government, where rather different views of public health prevails.

The issue, surely, is to be clear about those situations where, and tasks for which, a clinical input is necessary and to ensure that the appropriate expertise is available - that is, on tap if not on top.

The time may have come, therefore , to consider whether within public health medicine there is not a case to be made for specialisation so that its practitioners are required to choose between remaining guardians of the public's health - in which they will compete with other professionals who are not clinicians by training - or pursuing a rather narrower agenda which plays directly to their clinical expertise.

Indeed, if the issue continues to be fudged and the pretence persists that public health medicine can do it all - a sort of catch-all for the 'wicked problems' which nobody else wants - then the risk must be that it will do none of it. The public health function review started by the former chief medical officer, Sir Kenneth Calman, was expected to get to grips with the challenges and dilemmas facing public health, and in particular those in public health medicine occupying positions of leadership. It is important that this capacity strengthening exercise is not lost from view.

This tension between the broader health agenda and its clinical quality role may be the ultimate crossroads at which the specialty of public health medicine finds itself. It therefore needs to be much clearer and focused about what can realistically be delivered and on what its particular skills should most appropriately concentrate.

Is it to be clinical governance, or its traditional core business of public health? Few in public health medicine dare pose the question, and even fewer are prepared to do so publicly for fear of weakening the speciality's position. But the issue can no longer be avoided.

David Hunter is professor of health policy and management at the Nuffield Institute for Health, Leeds University.