Recent years have seen exciting developments in virtually every aspect of electroencephalography (EEG)/ magneto-encephalography (MEG) research of emotion, ranging from innovation in data-recording techniques to novel experimental paradigms and sophisticated analysis techniques. A major advantage of electrophysiological time series is that they directly reflect neuroelectric processes rather than blood flow (such as functional magnetic resonance imaging (fMRI)) or metabolic processes (such as positron emission tomography (PET)), which makes them unique indices of neuronal activity. This chapter considers the specific properties of the neural population activity that underlies EEG/MEG recordings. EEG and MEG are safe and noninvasive procedures that are associated with minimum discomfort to the participant. The chapter discusses a few interesting issues that inform but also constrain the use of frequencydomain techniques in affective neuroscience research. Electrophysiological studies of emotion may move beyond a descriptive analysis of waveforms to consider the data as measures of in-vivo neurophysiology.