This chapter focuses on the neuroimaging of cataplexy using a case of a 68-year-old woman, who had suffered from narcolepsy since she was 15 years old, as an example. Her mean sleep latency during a multiple sleep latency test was 0.5 minutes, with three sleep onset rapid eye movement (REM) periods among the four naps. The patient underwent two 99mTc-ethylcysteinate dimer brain single-photon emission computed tomography (SPECT) studies during symptomatic and asymptomatic periods of cataplexy on two non-consecutive days. Symptomatic SPECT images were coregistered with asymptomatic images and both images were then co-registered with 3-dimensional magnetic resonance imaging (MRI). The normalized subtracted SPECT and MRI volumes were merged for visual analysis. A characteristic of human REM sleep is right-hemisphere activation, as shown by SPECT imaging and spectral electroencephalographic (EEG) analysis. The right hemisphere is also more activated during cataplexy than the left hemisphere.