The Fenton reaction was discovered over 120 years ago, yet our understanding of the complete reaction mechanism of the seemingly simple iron and hydrogen peroxide reaction (Fe + H2O2) remains unclear, thus limiting its full potential. In this work, the aim is to summarize the processes that pertain to the (photo) Fenton reaction. More specifically, this review does not consider previous work relating to the Advanced Oxidation Process (AOP)-mediated disinfection/decontamination of stream flows, as much work has successfully achieved this. Instead, this manuscript presents an overview of the other fields of environmental and medical applications of (photo)Fenton: i) surface pre-treatment (nano-particles functionalization or modification), ii) terrestrial and algal biomass (pre)treatment to enhance high value products' recovery, iii) cancer treatment (malignant tumor elimination) and iv) other medical uses (antibiotics development, wounds disinfection, root canal sterilization and teeth whitening). For each field of application, the focus lies on analyzing the conceptualization and framework of the (photo) Fenton process, the opportunities and limitations, as well as a series of related applications. To conclude, the work discusses the current limitations of the existing applications and future research avenues.