This state-of-the-art review links the experimental data into a cohesive and critical account of CNS regeneration. Research findings are discussed in terms of their relevance to one (or more) of thirteen hypotheses concerned with regeneration in the mammalian CNS. Research findings reviewed include: regeneration in developing mammals and in submammalian vertebrates, the use of transplants and/or pharmacological treatments, in vitro studies on neurotrophic and neurite promoting factors and their potential relevance to CNS regeneration in vivo, and in vitro studies on the types of glial cells that may be responsible for enhancing or suppressing axonal re-growth.