The behaviour of hydroxyl radicals (OH*) in various aqueous environments is crucial to understanding its role in various important reactions within or at the surfaces of water and ice. The OH* is a key chemical species that appears across a diverse range of fields such as cosmic and nuclear reactions, atmospheric chemistry, and the biomolecular mechanisms of aging and diseases such as cancer, for example. OH* has proven to be a very challenging species to investigate because of its highly reactive nature. Here I will report insights into the behaviour of the hydroxyl radical in water and in ice through extensive studies utilizing Car-Parrinello molecular dynamics simulations. The reactivity, stability and mobility of OH*, and its relationship to local structure, will be discussed. I will demonstrate that the hydrogen atom transfer between OH* and a water molecule has a relatively small free energy barrier and follows an apparent hybrid (electron-proton transfer) mechanism in which local structural fluctuations play a important role. Details of the reactions and interactions that can occur between two OH* in water will be presented, where the production of an aqueous oxygen atom, O(aq), in triple state is observed. I will also show that two-center three-electron (“hemi-bond”) interactions play a crucial role in the behaviour of OH* in water and ice, particularly when there is a constrained hydrogen-bonding environment. Results from direct simulations, from constrained and metadynamics, and from gas phase benchmarking calculations will be presented. Implications for effective potentials for modeling OH* in aqueous environments will also be discussed.