This chapter reviews relevant research on emotion, focusing on aspects of emotion that are most relevant to human factors design. The review begins with a discussion of basic emotion concepts, such as low-level positive and negative affect; emotion dimensions of arousal and valence; and prototypical emotions, such as anger, happiness, and fear. The interaction between emotion processing and cognitive processes, such as attention, perception, and decision making, is also discussed. After a review of the nature of emotion, the problem of measuring emotions is considered. Available techniques for emotion measurement are surveyed, and recommendations are made concerning which techniques should be used in different contexts and interactive settings. Novel measures are also introduced for assessing variability in emotional experience and for measuring emotional expressiveness. After laying the intellectual groundwork for understanding the nature of emotional interaction and its measurement, the chapter then shifts to exploring the implications of emotional processing for the design of interactive systems. Affective designs are reviewed in the following domains: consumer products, health care, and robots. The chapter concludes with presentation of an initial set of guidelines relating to the design of interactive systems that take into account emotional processing. The material presented in this chapter is highly relevant to human factors engineering because knowledge of emotional processing and how it affects the state of operators or users and their performance can lead to more usable, effective, efficient, and satisfying interaction.