This chapter reviews the genetics of sleep and its most widely used correlate, the electroencephalogram (EEG), in mice and humans. Monozygotic (MZ) and dizygotic (DZ) studies allow measurement of genetic and environmental contributions to a trait. Reverse genetic approaches involve isolation of candidate genes, use of transgenic models, and phenotypic analysis of mutant animals. The first quantitative trait locus (QTL) mapping study for sleep amounts identified several genomic regions associated with the amount of rapid eye movement (REM) sleep. For the identification of genes involved in sleep, large-scale analysis of gene expression by microarrays has been performed in rats and mice. Microarray studies allow better understanding of how gene expression changes as a function of duration of wakefulness. A mutagenesis screen in mice is underway and might turn out to be successful in finding major genes regulating sleep duration as well as EEG.