Digital monitoring of physiological signals can allow computer systems to adapt unobtrusively to users, so as to enhance personalised 'smart' interactions. In recent years, physiological computing has grown as a research field, and it is increasingly considered in diverse applications, ranging from specialised work contexts to consumer electronics. Working in this emerging field requires comprehension of several physiological signals, psychophysiological states or 'indices', and analysis techniques. The resulting literature encompasses a complex array of knowledge and techniques, presenting a clear challenge to the practitioner. The Psychophysiology Primer provides a foundational review of the field of psychophysiology to serve as a primer for the novice, enabling rapid familiarisation with the core concepts, or as a quick-reference resource for advanced readers. It places special emphasis on everyday human-computer interface applications, drawing a distinction from clinical or sports applications, which are more commonplace. This primer provides the reader with a framework of commonly understood terms associated with experiential constructs and physiological signals. Then, 12 short and precisely focused review sections describe 10 individual signals or signal sources and present two technical discussions of online data fusion and processing. A systematic review of multimodal studies is provided in the form of a reference table. It concludes with a general discussion of the application of psychophysiology to human-computer interaction, including guidelines and challenges.