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Sleep and sleep disorders are complex phenotypes with genetic, epigenetic, and environmental influences. Twin studies allow researchers to parse out how these factors influence variability in sleep outcomes such as sleep duration and quality, chronotype, and disorders such as insomnia, hypersomnia, sleep apnea, sleep-related movement disorders, and parasomnias. Twin studies assess the overlap in genetic influences for sleep variance and other medical and psychiatric disorders and allow exploration of gene-environment interactions. Longitudinal twin studies demonstrate how these interactions change over the course of a lifetime. In general, twin studies demonstrate that the heritability of common sleep measures such as duration, quality, and chronotype is about 30-50%; however, this can vary widely between samples according to age, sex, comorbidities, and geographic location. Similarly, the heritability of sleep disorders including insomnia, obstructive sleep apnea, and parasomnias is also around 30-50%, with higher estimates for disorders known to run in families, such as sleepwalking. Heritability estimates for medical and psychiatric problems including obesity, depression, post-traumatic stress disorder, and mortality are higher with short sleep duration (typically < 7 h/n), suggesting that short sleep activates disease-related gene expression. Significant genetic overlap exists between insomnia and issues such as obesity, chronic pain, depression, and psychosis. In this review, we describe the major findings of twin studies related to sleep and how they impact our understanding of this critical component of health and disease.