An extension campaign is a coordinated effort to inform many farmers in a relatively short period of time about an agricultural topic of widespread concern or interest. The aim is to achieve quick, large-scale change in farmer behaviour and practices through carefully choreographed efforts by different organisations, using a variety of communication channels. An extension campaign requires a sharp focus (Box 1) and a clear end point. It should deliver material benefits to farmers, whose needs and demands are paramount in shaping the campaign.The chosen topic should have realistic and achievable outcomes. Campaigns are well suited to tackling plant health problems, where concerted action is needed to mitigate risks and to scale up proven but underutilised technologies. Campaigns go beyond the limited scope of individual projects to promote technologies and innovations to farmers. To be effective, campaigns need a panoply of partners and people, especially those beyond agriculture. Mass media and influential citizens, for example, offer new ways to reach large-scale audiences.Campaigns are usually one-off events, though some may last for several months or longer. Whatever their length, all campaigns should complement rather than replace existing extension efforts, promoting practical, direct ways to improve agriculture and benefit livelihoods.Extension campaigns differ from advocacy campaigns, which aim to influence policy, for example on the use of genetically modified crops. The most successful campaigns think and act expansively, encourage wide participation, and focus on topics that matter most to people.