This article attempts to review briefly current opinions on Johne's disease, or paratuberculosis, in ruminants caused by Mycobacterium avium subsp. paratuberculosis. Paratuberculosis has been known to be prevalent in domestic livestock, such as cattle, goats, and sheep, for more than a century. Despite this knowledge only minor efforts have been made to control the disease and, with the attention being focussed on the eradication of other diseases, the problem of paratuberculosis has been neglected in most countries in the past decades. However, recent epidemiological surveys performed in Europe showed a high prevalence of paratuberculosis in cattle and sheep, indicating that the situation has become quite alarming. In addition, the possible role of M. avium subsp. paratuberculosis in the aetiology of Crohn's disease in humans is still debated, as discussed in this article. Therefore, there is suddenly a renewed interest in paratuberculosis, and the disease is recognized as a significant problem. As a consequence, there is a need for reliable diagnostic tools for large-scale use to allow the introduction of programmes to control and eventually eradicate the disease. The current status and the possibilities for such programmes are discussed.