The article provides a comprehensive review of ultra-weak photon emission (UPE), a phenomenon where biological systems emit photons at extremely low intensities. The research history of UPE is traced back to Alexander Gurwitsch's 1920s experiments, which suggested non-chemical cell-to-cell communication through "mitogenic radiation." Despite initial skepticism and challenges, the field has seen resurgence with studies demonstrating UPE in various biological systems, including bacteria, fungi, and animal tissues. UPE is primarily generated by metabolic reactions involving reactive oxygen species (ROS) and can be detected using specialized detectors such as electron-multiplying charge-coupled devices (EMCCDs) and photomultiplier tubes (PMTs). The article discusses the challenges in detecting UPE, including the need for specialized equipment and the presence of background noise. Applications of UPE are explored in plant biology, food quality, environmental monitoring, disease diagnosis, and brain function, highlighting its potential as a non-invasive diagnostic tool. However, the low intensity of UPE and the difficulty in distinguishing it from other light sources remain significant hurdles. The authors conclude by emphasizing the importance of further research to understand the role of UPE in biological systems and its potential applications.The article provides a comprehensive review of ultra-weak photon emission (UPE), a phenomenon where biological systems emit photons at extremely low intensities. The research history of UPE is traced back to Alexander Gurwitsch's 1920s experiments, which suggested non-chemical cell-to-cell communication through "mitogenic radiation." Despite initial skepticism and challenges, the field has seen resurgence with studies demonstrating UPE in various biological systems, including bacteria, fungi, and animal tissues. UPE is primarily generated by metabolic reactions involving reactive oxygen species (ROS) and can be detected using specialized detectors such as electron-multiplying charge-coupled devices (EMCCDs) and photomultiplier tubes (PMTs). The article discusses the challenges in detecting UPE, including the need for specialized equipment and the presence of background noise. Applications of UPE are explored in plant biology, food quality, environmental monitoring, disease diagnosis, and brain function, highlighting its potential as a non-invasive diagnostic tool. However, the low intensity of UPE and the difficulty in distinguishing it from other light sources remain significant hurdles. The authors conclude by emphasizing the importance of further research to understand the role of UPE in biological systems and its potential applications.