Ultra weak photon emission (UPE) is a phenomenon where cells emit light at very low intensities, distinct from other light emission processes like bioluminescence. UPE is also known as biophoton, biological autoluminescence, and ultraweak photon emission. It is not inherently "weak" or biological in nature, but rather a byproduct of cellular metabolism. Research on UPE has a long history, but has been hindered by the lack of sensitive detection technology. Recent advances in technology have made it easier to detect and image UPE, as well as to understand its function. This review discusses the history of UPE research, its proposed mechanisms, possible biological roles, detection methods, and potential medical applications. UPE is produced during essential metabolic reactions, characterized by molecules moving from high to lower energy states, releasing photons and electronically excited products. The main mechanism of UPE is the production of reactive oxygen species (ROS), which are derived from the stepwise reduction of molecular oxygen. UPE may also arise from the breakdown of reactive nitrile species and the general cessation of electronically excited states. The nature of ROS and their association with biophoton production is a key consideration. ROS are unavoidably produced by energy generating pathways, including photosynthesis, glycolysis and certain facets of mitochondrial respiration. High levels of ROS are generally toxic due to their formidable capacity to oxidise metabolic products including carbohydrates, proteins, lipids and DNA, whilst excessive ROS build up can eventually lead to a breakdown in normal metabolic function and ultimately cell death. UPE detection is challenging due to the extremely low intensity of the emission. The greatest challenge in this field is the satisfactory detection and characterization of photons produced by intracellular metabolic processes. The intensity of such emission is estimated to range from tens to a maximum of one hundred photons s⁻¹ cm⁻² and span the visible to NIR regions of the spectrum. The detection of UPE is complicated by the presence of extrinsic light sources, including daylight, room light and light from instruments. These can be eliminated by having a chamber or box which is completely sealed, as well as performing experiments in rooms with zero light. However, this is still insufficient to eliminate cosmic rays, radioactive decay and other high energy sources of photons, thus experimental design needs to include management of these artifacts. The main method for UPE detection, PMTs, are still the best way of getting quantitative counts of photons from samples. However, with the development of electron-multiplying technology we can sacrifice precise photon counts for imaging signal above the noise level and many new camera technologies are achieving this. Careful choice of scientific question and experimental design are the most essential when trying to measure UPE. UPE has potential applications in various fields, including plant biology, food quality, environment and pollutants, disease and drug development, and brain function. In plant biology, UPE is tightly coupled to the physiological state of a plant, and changesUltra weak photon emission (UPE) is a phenomenon where cells emit light at very low intensities, distinct from other light emission processes like bioluminescence. UPE is also known as biophoton, biological autoluminescence, and ultraweak photon emission. It is not inherently "weak" or biological in nature, but rather a byproduct of cellular metabolism. Research on UPE has a long history, but has been hindered by the lack of sensitive detection technology. Recent advances in technology have made it easier to detect and image UPE, as well as to understand its function. This review discusses the history of UPE research, its proposed mechanisms, possible biological roles, detection methods, and potential medical applications. UPE is produced during essential metabolic reactions, characterized by molecules moving from high to lower energy states, releasing photons and electronically excited products. The main mechanism of UPE is the production of reactive oxygen species (ROS), which are derived from the stepwise reduction of molecular oxygen. UPE may also arise from the breakdown of reactive nitrile species and the general cessation of electronically excited states. The nature of ROS and their association with biophoton production is a key consideration. ROS are unavoidably produced by energy generating pathways, including photosynthesis, glycolysis and certain facets of mitochondrial respiration. High levels of ROS are generally toxic due to their formidable capacity to oxidise metabolic products including carbohydrates, proteins, lipids and DNA, whilst excessive ROS build up can eventually lead to a breakdown in normal metabolic function and ultimately cell death. UPE detection is challenging due to the extremely low intensity of the emission. The greatest challenge in this field is the satisfactory detection and characterization of photons produced by intracellular metabolic processes. The intensity of such emission is estimated to range from tens to a maximum of one hundred photons s⁻¹ cm⁻² and span the visible to NIR regions of the spectrum. The detection of UPE is complicated by the presence of extrinsic light sources, including daylight, room light and light from instruments. These can be eliminated by having a chamber or box which is completely sealed, as well as performing experiments in rooms with zero light. However, this is still insufficient to eliminate cosmic rays, radioactive decay and other high energy sources of photons, thus experimental design needs to include management of these artifacts. The main method for UPE detection, PMTs, are still the best way of getting quantitative counts of photons from samples. However, with the development of electron-multiplying technology we can sacrifice precise photon counts for imaging signal above the noise level and many new camera technologies are achieving this. Careful choice of scientific question and experimental design are the most essential when trying to measure UPE. UPE has potential applications in various fields, including plant biology, food quality, environment and pollutants, disease and drug development, and brain function. In plant biology, UPE is tightly coupled to the physiological state of a plant, and changes