The policy literature often mentions the agenda-setting influence of focusing events, but few policy studies systematically examine the dynamics of these events. This article closes this gap by examining focusing events, group mobilization and agenda-setting. Using natural disasters and industrial accidents as examples, most focusing events change the dominant issues on the agenda in a policy domain, they can lead to interest group mobilization, and groups often actively seek to expand or contain issues after a focusing event. I explain how differences in the composition of policy communities and the nature of the events themselves influence group and agenda dynamics. The organization of policy communities is an important factor in agenda setting, but agenda setting and group politics vary considerably with the type of event and the nature of the policy community.