Monooxygenase (MO) enzymes are responsible for the oxidation of hydrocarbons and other compounds in the carbon and nitrogen cycles, are important for the biodegradation of pollutants and can act as biocatalysts for chemical manufacture. The soluble di-iron monooxygenases (SDIMOs) are of interest due to their broad substrate range, high enantioselectivity and ability to oxidise inert substrates such as methane. Here, we re-examine the phylogeny and functions of these enzymes, using recent advances in the field and expansions in sequence diversity in databases to highlight relationships between SDIMOs and revisit their classification. We discuss the impact of horizontal gene transfer on SDIMO phylogeny, the potential of SDIMOs for the biodegradation of pollutants and the importance of heterologous expression as a tool for understanding SDIMO functions and enabling their use as biocatalysts. Our analysis highlights current knowledge gaps, most notably, the unknown substrate ranges and physiological roles of enzymes that have so far only been detected via genome or metagenome sequencing. Enhanced understanding of the diversity and functions of the SDIMO enzymes will enable better prediction and management of biogeochemical processes and also enable new applications of these enzymes for biocatalysis and bioremediation.