Cell membranes are composed of a lipid bilayer containing proteins that span the bilayer or interact with lipids on either side. Recent advances in lipid analysis reveal that eukaryotic cells contain hundreds of lipid species, but the function of this diversity remains unclear. The lipid bilayer, with its two leaflets, forms a two-dimensional liquid with unique properties that enable cellular functions. This bilayer can segregate its components laterally, a process based on dynamic liquid-liquid immiscibility, leading to the raft concept of membrane subcompartmentalization. This principle combines the self-assembly of sphingolipids and cholesterol with protein specificity to regulate membrane bioactivity. All cells are enclosed by membranes that define their spatial identity and separate intracellular and extracellular environments. These membranes consist of lipids and proteins, with the hydrophobic and hydrophilic properties of lipids enabling the spontaneous formation of the lipid bilayer. This amphipathic nature allows cells to segregate internal components from the external environment. Lipids can form different structural organizations, known as lipid polymorphisms, depending on their shape and composition. The basic structure of all cell membranes is the lipid bilayer, which is a fundamental molecular model of cellular structures. Cellular membranes are not dominated by lipids but are packed with transmembrane proteins. These membranes are dynamic, with interactions between membrane proteins and lipids, and have compositional asymmetry, requiring significant energy to maintain. Eukaryotic membrane lipids include glycerophospholipids, sphingolipids, and sterols. Sterols, such as cholesterol, are crucial for membrane trafficking and are involved in the formation of lipid rafts. The complexity of lipid composition is essential for maintaining membrane stability and function, especially in response to changes in composition, osmolarity, or pH. Sterols and sphingolipids play a key role in the biosynthetic pathway from the endoplasmic reticulum (ER) to the plasma membrane (PM). The cholesterol gradient helps organize this pathway, with cholesterol content increasing from the ER to the PM. This gradient is important for the sorting of proteins and lipids, ensuring proper membrane trafficking. In the secretory pathway, sorting of both proteins and lipids occurs before exit from the trans-Golgi network. The presence of separate pathways from the Golgi complex indicates that hydrophobic matching is not the only principle involved. Sterols and sphingolipids are enriched in the trans-Golgi region, facilitating the sorting of proteins and lipids. In yeast and MDCK cells, there are distinct pathways for membrane transport, with lipid sorting occurring in the trans-Golgi network (TGN). Lipidomic analysis shows that sterols and sphingolipids are enriched in the plasma membrane (PM), indicating that lipid sorting occurs in the TGN. This sorting is crucial for the formation of apical and basCell membranes are composed of a lipid bilayer containing proteins that span the bilayer or interact with lipids on either side. Recent advances in lipid analysis reveal that eukaryotic cells contain hundreds of lipid species, but the function of this diversity remains unclear. The lipid bilayer, with its two leaflets, forms a two-dimensional liquid with unique properties that enable cellular functions. This bilayer can segregate its components laterally, a process based on dynamic liquid-liquid immiscibility, leading to the raft concept of membrane subcompartmentalization. This principle combines the self-assembly of sphingolipids and cholesterol with protein specificity to regulate membrane bioactivity. All cells are enclosed by membranes that define their spatial identity and separate intracellular and extracellular environments. These membranes consist of lipids and proteins, with the hydrophobic and hydrophilic properties of lipids enabling the spontaneous formation of the lipid bilayer. This amphipathic nature allows cells to segregate internal components from the external environment. Lipids can form different structural organizations, known as lipid polymorphisms, depending on their shape and composition. The basic structure of all cell membranes is the lipid bilayer, which is a fundamental molecular model of cellular structures. Cellular membranes are not dominated by lipids but are packed with transmembrane proteins. These membranes are dynamic, with interactions between membrane proteins and lipids, and have compositional asymmetry, requiring significant energy to maintain. Eukaryotic membrane lipids include glycerophospholipids, sphingolipids, and sterols. Sterols, such as cholesterol, are crucial for membrane trafficking and are involved in the formation of lipid rafts. The complexity of lipid composition is essential for maintaining membrane stability and function, especially in response to changes in composition, osmolarity, or pH. Sterols and sphingolipids play a key role in the biosynthetic pathway from the endoplasmic reticulum (ER) to the plasma membrane (PM). The cholesterol gradient helps organize this pathway, with cholesterol content increasing from the ER to the PM. This gradient is important for the sorting of proteins and lipids, ensuring proper membrane trafficking. In the secretory pathway, sorting of both proteins and lipids occurs before exit from the trans-Golgi network. The presence of separate pathways from the Golgi complex indicates that hydrophobic matching is not the only principle involved. Sterols and sphingolipids are enriched in the trans-Golgi region, facilitating the sorting of proteins and lipids. In yeast and MDCK cells, there are distinct pathways for membrane transport, with lipid sorting occurring in the trans-Golgi network (TGN). Lipidomic analysis shows that sterols and sphingolipids are enriched in the plasma membrane (PM), indicating that lipid sorting occurs in the TGN. This sorting is crucial for the formation of apical and bas