TITLE: Machine Learning for Social Systems: Modeling Opinions, Activities, and Interactions ABSTRACT: The proliferation of user-generated content on the web provides a wealth of opportunity to study human behavior through users' online traces. My research aims to model such behavior through the lens of opinions. Opinions are ubiquitous online: people share opinions explicitly in the form of ratings, reviews, and "likes"; and implicitly, through the products they purchase, the connections they form, and the communities they join. Such opinions are much more than a simple number or quantity: they are multi-dimensional, they develop over time, and they are influenced by our friends and communities. In this talk, I will present machine learning techniques in order to model and understand such rich, structured, and time-evolving data.