Affective user interfaces are interfaces that are capable of eliciting, conveying, modeling, enhancing, or influencing emotions in their user. This chapter summarizes the role user affect plays in interface design, including how it can best be understood and represented, and the variety of methods pertaining to its analysis and display. Drawing on the state of the art and history of affective interfaces, we highlight how such interfaces can be used to enhance existing computer-mediated communication to make them more engaging and more natural, as well as to enable new interaction possibilities. Specifically, we focus on: (1) augmenting computer-mediated communication with affect, (2) digital emotion regulation and support, (3) affective immersive experiences, (4) affective haptics, and (5) persuasive interfaces. Finally, we consider the risks of these technologies, including ethical aspects (e.g., emotion surveillance, ground truth reliability, and bias), as well as the opportunities for such interfaces, from affective embodied agents designed for health and positive behavior changes, to affective learning and education, and to artistic creations.