Lipids and derivatives (lipoproteins, glycolipids, phospholipids) are of crucial importance for cell structure and function. Biological membranes provide a selective barrier controlling the passage of all kinds of molecules, both between cells and extracellular space and between different compartments within the cells. Membranes are the turntables for crucial processes in neurobiology, muscle contraction, and cell signaling. Lipophilic fluorescent dyes are also suitable for preparation of optical sensors, polymer sciences, and other research areas. Lipophilic fluorescent probes are useful tools for: investigation of lipid metabolism, imaging cell structures, analysis of biophysical processes, and investigation of cell signaling processes. Sigma offers a wide range of lipophilic probes, including: fluorophore labeled phospholipids, labeled lipopolysaccharides and steroids, membrane permanent reactive tracers, environmental sensitive membrane probes, and many other lipophilic probes