Lipids are structural components of cellular membrane and functional components exerting an important role in different metabolic pathways through regulation of cell signaling and metabolic homeostasis. Although the human body can synthesize some fatty acids, essential fatty acids (EFA) should be exogenously provided. Parenteral nutrition (PN) can be utilized to provide EFA for patients without normal gastrointestinal functions and those at risk of malnutrition. Parenteral lipids provide more energy-dense nutrition in a reduced volume and prevent EFA deficiency. However, prolonged PN with lipid emulsions is associated with metabolic complications including PN-associated liver disease. Currently available parenteral lipid emulsions provide various combinations of soybean oil, fish oil, olive oil and medium-chain triglycerides. The effects of parenteral lipid emulsions on the lipid nutrition status and PN-associated complications vary depending on the type and dosages of main oil sources used in parenteral lipid solutions. Therefore, understanding the composition, metabolism, and functional effects of lipids can be helpful in considering their clinical applications in PN.