Recent advances in research on intellectual and developmental disabilities include the development of innovative methods to understand the developmental pathways that lead to disabilities. To illustrate these new developments, this entry summarizes key issues regarding the definition of disabilities and then describes the strengths and weaknesses of three primary methods that have been used to assess stability and change in disabilities across development. The final two sections of this entry describe some of the innovative methods [Page 412]that can be used to identify the causal factors and neural processes that lead to the development of intellectual and developmental disabilities.