This chapter briefly reviews the literature on developmental plasticity and what is known about sensitive periods for language development, with specific reference to the implications for paediatric cochlear implantation. The data show that the earlier that profoundly deaf children receive a cochlear implant, the better the outcome, whether it be speech perception or production. Age at implantation, however, is not the only important factor in determining outcome, and other factors play as important a role. Some of these other factors are identified, for example, communication mode, educational setting, quality of acoustic environment and degree of residual hearing. The challenge for research is to study large enough numbers of children to show clear and systematic effects for each of these, and other, factors. Large numbers are required to perform appropriate statistical analysis, and so it is argued that cochlear implant centres need to standardise on surgical, audiological, psychological, speech and language therapy and education outcome measures to enable pooling of data.