Mitochondrial respiration and glycolytic activity are the main energy metabolic pathways in cells. Analyzingcellular energy metabolism is important for understanding the state of cells. The extracellular flux analyzer (XF)is a cell metabolism analyzer that measures the oxygen consumption rate (OCR) and the extracellular acidificationrate (ECAR) of cells in extracellular medium to profile the activities of mitochondrial respiration and glycolysis,respectively. This device is a system that can noninvasively measure metabolism over time on 24- to 96-well culture plates. The XF-specific sensor cartridge has a fluorescent dye embedded in the tip that serves as an oxygensensor and pH sensor. The mortar-like shape of the bottom of the cell culture plate creates a microspace of 7 μL orless around the cells. This microscopic space makes it possible to measure minute changes in the culture mediumdue to cellular energy metabolism. By adding various compounds to the drug port attached to a cartridge equippedwith a fluorescent sensor, it becomes possible to measure cellular metabolism in real time. The use of XF to elucidate cellular energy metabolism mechanisms in various research fields, such as cancer, metabolic diseases, andimmunity, may lead to the development of new testing methods and treatments.