Once a new commercial product, technology, political opinion, or social norm is adopted by a few people, these few often put peer pressure on others to consider adopting it as well. Those who adopt next put even more pressure on the rest of the population. This cascading “epidemic” effect is often called diffusion in social networks. There are many natural questions that can be asked about diffusion. Which initial group of people should get “infected” by a new product to ensure its adoption by the largest possible group? Which group should be convinced that an idea is bad, in order to avoid its wide spread? How does marketing affect the diffusion? In this talk I will introduce the most commonly used mathematical model of diffusion and talk about several of my papers on logical systems that capture properties of this model.