In 2007, a group of accomplished scholars gathered in Ithaca, New York, to discuss diffusion and social movements, producing the initial drafts of the chapters that appear in this most welcome volume. Events in the world since then have underscored how important the ideas developed here are in understanding the dynamics not only of social movements but also of contemporary politics.

For example, in the summer of 2009, Tea Party activists appeared at town meetings about health care, shouting down their elected officials with scripted chants that had been distributed by a conservative political organization, FreedomWorks. Across the United States, activists chanted “read the bill” and ran through similar shouting points. This is an example of what Sidney Tarrow, in the conclusion to this book, describes as “relational diffusion,” in which ideas, tactics, and/or issues are transmitted by direct contact.

Not too long afterward, in December 2010, in an unexpected response to the self-immolation of a Tunisian fruit vendor, activists assembled in street protests to demand political reform. Within two months, they had driven Tunisian President Zine El Abidine Ben Ali from office, inspiring people in more than a dozen other Middle Eastern countries to stage their own campaigns for regime change. The protesters learned of activities elsewhere through both mass media and social media, and improvised their own scripts and claims—a wave, in Tarrow's terms, of “nonrelational diffusion.”

Less than a year later, in response to a call for action in a Canadian anticonsumer magazine, activists in New York City assembled downtown to “Occupy Wall Street.” Over the course of a few weeks, Occupy campaigns spread to more than 2,000 sites, mostly but not exclusively in the United States. Each campaign was organized locally, with distinct supporting coalitions and issues, but borrowing the appellation “Occupy,” as well as some of the rhetoric and procedures of the Wall Street site. Some transmission was indirect, through public media, but some ideas were transmitted along well-established organizational networks—a case with both relational and nonrelational mechanisms at work.

These examples illustrate that diffusion is both a long-standing and a contemporary issue of critical importance to social movement campaigns. In their introductory essay, the editors offer a useful summary of the questions such episodes raise, highlighting that we want to understand what diffuses, how it does so, and the impact of these diffusion processes on both movement dynamics and social change more broadly. The nine central chapters, bookended by the extremely useful introduction and Tarrow's sharp conclusion, take very different approaches to these and related questions, exploring a wide range of empirical materials but providing a range of useful conceptual tools and empirical findings for the reader.

It is best to read from the outside in, starting with the introduction and conclusion, to get a theoretical foundation for exploring the range of empirical investigations. The introduction gives us a clear set of analytical questions to use in investigating any range of cases, urging the reader to consider carefully just what is being diffused, noting the analytical payoff of separating tactics, claims, and frames or rhetoric. Tactics, for example, can diffuse not only across boundaries but across the ideological spectrum, from one movement to another. There is a great deal of important work that traces the spread of particular tactics across different contexts over time, from innovators to receptors.

While some tactics, like the mass demonstration, are quickly divorced from the intentions of the initiator, others are more closely linked to a particular set of actors and claims. American college students opposed to apartheid in South Africa built shantytowns, that is, semipermanent encampments, on their campuses in the 1980s. Sarah Soule's early work detailed the diffusion of the tactic, which largely disappeared for two decades until an odd reemergence in the Occupy protests.

Tarrow's conclusion offers a brief for moving beyond detailing the patterns of diffusion to explore the how, with a focus on a mechanism-based approach to analyzing political contention. This means, he contends, paying attention to the interactions between authorities and challengers as a protest spreads (or is contained), and as it shifts across venues and targets. He offers a conceptual kit that contains far more tools than any of us will use in any specific investigation.

The empirical works in the middle of the book offer exemplary treatments of the broad range of useful work on diffusion that is possible, and together offer even more for that tool kit. The first part focuses on the diffusion of movement frames, that is, interpretive schema that make sense of a movement's demands. In investigating the development of antievolution activism in the United States, for example, James Stobaugh and David Snow show how authorities, in this case the Supreme Court, constrained the diffusion of some frames by explicitly rejecting them. Diffusion is predicated on frame innovation. Lance Compa's chapter on framing labor claims as human rights emphasizes the strategic dimension of constructing issues, as organizers try to find ways to define their cause that will spread more readily. Ronald Herring shows the critical role of experts in validating and projecting a set of ways to make claims about genetically modified organisms, identifiying a two-way process of diffusion, bottom-up and top-down.

Part II focuses on the mechanisms of diffusion, and the chapters are rich with historical detail about particular campaigns. Sean Chabot traces personal contacts and conversations to explain the spread of Mahatma Gandhi's ideas about strategic nonviolence from India to the United States in the 1930s, and developing a visible profile decades later as articulated—and reframed—by Martin Luther King, Jr. (The paperback edition of the book features a photograph of Dr. King with Gandhi's portrait hanging in the background.) The modern contrast is offered by Jennifer Earl and Katrina Kimport, who present the leading edge of new research on online activism. In this chapter, they focus on Websites that can offer information and/or host online actions, finding that well-established groups often sponsor Websites that provoke action online—and offline as well. Valerie Bunce and Sharon Wolchik argue that the spread of elections that toppled authoritarian governments in former communist states in Central and Eastern Europe reflected diffusion dynamics at both activist and state levels. They explain that activists emulate what seems to be successful elsewhere when they can see those initiators as similar to themselves. Recognizing similarities involves some constructive work; Occupy Wall Street activists frequently cite the protesters in Egypt's Tahrir Square as inspiration, even though there is no evidence that the demonstrators in Zuccotti Park were waiting for the army to take sides as they did in Egypt.

One empirical chapter focuses on the outcomes of diffusion, and it is a signal example. Michael Biggs and Kenneth Andrews revisit the American sit-in movement of the early 1960s, tracing the growth and spread of a tactic and the concomitant growth of Civil Rights organizations. The famous first sit-in in Greensboro, North Carolina, was staged with little preparation by four students without the endorsement of any organization; the spread was possible only when existing Civil Rights networks pitched in to publicize the event and mobilize other activists. The campaign established a new organization, the Student Nonviolent Coordinating Committee, but it would prove to be short-lived, with existing organizations like the NAACP left to speak for the efforts of the activists on the front lines.

The limited organizational impact of a protest campaign that captured the popular imagination and promoted real legislative change is one of the many provocative puzzles that this important volume raises for the rest of us.