A variety of spectroscopic techniques are being used to investigate the electronic structure and reactivity of sulfur-based radicals in peptides, proteins, and enzymes. Nature uses these radicals in many ways, including for catalysis (e.g. ribonucleotide reductases), for protection against oxidative stress (e.g. glutathione), and even possibly for redox signalling. There is also speculation that such radicals are crucial intermediates involved in oxidative neurodegeneration in Alzheimers' disease. A systematic characterisation of sulfur-centred radicals in model proteins and peptides is currently underway to understand the effect of the surrounding medium on their reactivity. These studies are simultaneously being extended to specific enzymatic systems to allow us to better comprehend the exact nature of the reactive species in actual biological systems.