Positive emotions are an important part of human life. They can be very strong, for example when we fall in love, land our dream job, or attend a concert or sports event. But more often they are less intense, for example when enjoying a meal or watching a sunset on a beautiful summer evening. Since the seminal work in the 1990s by Barbara Fredrickson, Martin Seligman and others in the 1990s, scientific research has made increasingly clear that positive affective states are not just mere epiphenomena but that they may have clear functional roles that are strongly rooted in our biological makeup. This special issue focuses on positive affect, an umbrella term that contains both specific emotions, such as gratitude, pride, and interest, as well as more long-term and diffuse affective states, such as happiness and content moods. For example, empirical research has shown that positive affect plays a role in behavioural flexibility, reducing stress, and facilitating psychosocial functioning. Recent work further indicates that understanding disturbances of positive affect can aid the diagnosis and treatment of mental disorders. However, despite the growing appreciation of the importance of positive affect in the fields of psychology and neuroscience, many questions still remain: What are positive emotions, how are they generated in the brain, how do they relate to and influence cognition, and how can this knowledge can be applied to improve mental and physical health, and well-being?In March 2020, the editors of this special issue together with Siri Leknes (University of Oslo), brought a small group of researchers together in Leiden, the Netherlands to study these questions. They included emotion scientists, social and cognitive psychologists, and neuroscientists, each approaching positive affect from their own discipline’s perspective. At the Lorentz Center we learned about each other’s work and engaged in intense discussions with the aim to achieve cross-fertilization in this area. The insights gained yielded many of the papers in this special issue. Moreover, the event initiated several papers in which participants from different fields joined forces to produce interdisciplinary contributions to this special issue. In addition, we invited other experts and leaders in the field to contribute. Together, this results in a rich overview of research on positive affect, with contributions from a variety of different perspectives in psychology and neuroscience.The current issue consists of 34 contributions that are divided into three overarching topics. Part I is concerned with the question of what constitutes positive affect and emotion. This section includes papers that introduce possible taxonomies of positive affect, and that differentiate between types of positive emotions and positive affective states, as well as their different behavioral and social functions. Part II focuses on mechanisms that generate or involve positive affect, identifying those at multiple levels of the brain, the body and behaviour, and showing how positive affect relates to and influences cognition and motivation. Finally, part III focuses on papers that describe applications of positive affect research in the domains of stress, health, and psychopathology.