Apolipoprotein E (apoE) is a key apoprotein in lipid transport and is susceptible to genetic mutations. ApoE variants have been studied for four decades and more than a hundred of them have been reported. This paper presents an up-to-date review of the function and structure of apoE in lipid metabolism, the E2, E3, and E4 isoforms, the APOE gene, and various pathologies, such as familial type III hyperlipidemia and lipoprotein glomerulopathy, caused by apoE variants. Alzheimer's disease was barely mentioned in this paper. But this review should help researchers obtain a comprehensive overview of human apoE in lipid metabolism.