This chapter reviews the clinical and medical aspects of Angelman syndrome (AS) and the genetic mechanisms that create a deficiency of the function of UBE3A on the maternally-derived number 15 chromosome. Genetic mechanisms causing AS include cytogenetic deletions, imprinting center defects, paternal uniparental disomy and UBE3A mutations. We review the clinical criteria for the diagnosis of AS and discuss genetic confirmatory testing. The chapter explores the natural history and characteristic features of AS including severe developmental delay, speech impairment, gait ataxia, microcephaly, seizures, and unique behaviors. Finally, we will discuss the implications for learning and overall function.