The classification of sleep disorders is essential both for correct diagnosis and for coding purposes. There are three major sleep disorder classifications in the USA: the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V), the American Academy of Sleep Medicine’s International Classification of Sleep Disorders (ICSD-3), and the International Classification of Diseases Modified Version (ICD-10-CM). This chapter discusses these classifications and their differences. DSM-V and ICSD-3 are used mainly for diagnostic information, whereas ICD-10-CM is used for coding. Sleep disorders can be regarded as falling into three main groups: those that cause difficulty with nighttime sleep, those that cause daytime sleepiness, and those that cause abnormal behavior during the night. The disorders, such as insomnia disorder, narcolepsy, circadian rhythm sleep disorders, obstructive sleep apnea syndrome, and the parasomnias, in these three groups are organized differently, depending upon the classification system. The most detailed diagnostic classification system is ICSD-3, whereas the DSM-V classification is a simplified version, predominantly for psychiatrists.