This project explores cohort and period trends in political participation in the United States between 1973 and 2008. We examine the extent to which protest attendance and petition signing have diffused to different kinds of actors across multiple generations; we test claims central to understanding trends in social movement participation. Using aggregated, cross- sectional survey data on political involvement from 34,241 respondents, we examine changes in the probability of ever having attended a protest or signed a petition over time periods and across cohorts using cross-classified, random-effects models. We find a strong generational effect on the probability of ever having attended a protest, which explains much of the observed change in self-reports of protest behavior. More than half of this generational effect is a result of compositional change, but we find little evidence that protest attendance dif- fused to new types of actors. We compare these findings with a less confrontational form of protesting, petition signing, which shows more period than cohort effects. We argue that social movement activities have not become a widespread means of civic engagement.