We at the Center of Excellence in Climate Change Communication Research make the case that there are two fundamental options for influencing peoples’ behavior en mass. One can influence people directly, for example, by changing what they think and feel, changing how they interact with the other people in their social network, and/or changing how they function collectively as a community. Alternatively, one can influence people indirectly by changing the attributes of the environments in which people live (such as the cost and availability of products and services in their communities, and the public policies of their town, state and nation). Each approach has potential to create large-scale behavior change; used together, these approaches have even greater potential to shape population behavior.