Multiple lines of evidence implicate redox-active transition metals as mediators of oxidative stress in neurodegenerative diseases. Among the recent research discoveries is the finding that transition metals bind to proteins associated with neurodegeneration, including the prion protein. Whereas binding in the latter case may serve an antioxidant function, adventitious binding of metals to other proteins appears to preserve their catalytic redox activity in a manner that disturbs free radical homeostasis. Alterations in the levels of copper- and iron-containing metalloenzymes, involved in processing partially reduced oxygen species, are also likely to contribute to altered redox balance in neurodegenerative diseases. Nonetheless, even in familial forms of amyotrophic lateral sclerosis linked to mutations in superoxide dismutase, it is unclear whether an altered enzyme activity or, indirectly, a disturbance in transition-metal homeostasis is involved in the disease pathogenesis.