Khat has been used as a stimulant plant in many parts of Africa and the Arabian Peninsula for centuries. Its current use among particular migrant communities in Europe and elsewhere has caused alarm among policy makers and health care professionals. In the United Kingdom, the debate over the psychiatric and social implications of khat use has led to a demand for stricter legal control of this stimulant plant. This paper (a) provides a historical overview of khat use, and (b) reviews the evidence for the existence of a causal link between khat use and mental illness. To do so, we undertook a detailed search of social and medical science databases for case reports, qualitative and quantitative articles on khat use and mental illness from 1945 to 2006. The validity and reliability of the studies that met our inclusion criteria were examined. Lastly, although highlighting health concerns about khat use we suggest that the debate about this popular drug in migrant populations carries elements of a 'moral panic'. There is a need for improved research on khat use and its possible association with psychiatric disorders.