From a public health perspective, there is probably no more important or daunting challenge than reducing the prevalence of the most common mental disorders, namely anxiety and depression. These disorders have a combined community prevalence rate of between 15% and 30% (Cox et al. 1987; Robins et al. 1991; Goldberg & Huxley, 1992; Kessler et al. 1994; Meltzer et al. 1995) and account for one-third of days lost from work due to ill health (Jenkins, 1985a) and one-fifth of all consultations in general practice in the UK (Williams et al. 1986). The common mental disorders are associated with impairments in physical and social functioning at least as severe those associated with chronic physical illnesses (Wells et al. 1988; Klerman, 1989; Wohlfarth et al. 1993; Ormel & Costa e Silva, 1995; Spitzer et al. 1995; Martin et al. 1996), and a mortality rate nearly twice that of the general population (Murphy et al., 1987; Klerman, 1989; Lloyd et al. 1996). The public health importance of these disorders, even in mild form, is further demonstrated by the finding that low levels of depression resulted in 51% more days lost from work than major depression (Broadhead et al. 1990). The total annual cost of the common mental disorders in the UK may amount to £6 billion, of which two-thirds arises from lost productivity (Croft-Jefferys & Wilkinson, 1989). Despite the availability of simple, cheap and effective treatments (Paykel & Priest, 1992; Effective Health Care, 1993; Brugha, 1995), one study found that the prevalence of these disorders in Britain may have increased recently (Lewis & Wilkinson, 1993).