INTRODUCTION The Rodent Models Core provides technical support for the generation and characterization of transgenic and knockout strains of mice, access to expensive behavioral equipment for mice and rats that can be shared among several investigators, and expertise in the behavioral phenotyping of mouse and rat models of MRDD and related disorders of the CNS. In addition, the Core is responsible for the care, feeding, and recordkeeping related to the use of mice and rats by MRDDRC projects and the maintenance of specific rodent models of development, developmental disabilities, and neurodegenerative diseases. This Core has been renamed during the present project period, reflecting a sharpening of its focus and considerable new expertise and services added during the past five years in order to be both responsive to the growing and changing needs of MRDDRC investigators and to contribute to the evolving scientific agenda of the MRDDRC. The space allocated to this Core is in the midst of a major renovation that, as of January 2005, resulted in the temporary relocation of all animals to other facilities on campus, with reassignment of the caretaker staff to those facilities. While renovation is taking place, Core services are continuing uninterrupted at those alternate locations. This description of the Rodent Models Core includes an explanation of how this Core will operate when the animals return in 2007, with newly trained staff who will operate this exceptional and fully modernized facility serving the diverse scientific needs of MRDDRC investigators using rodent models. In the description below, the present tense is used to portray resources and services that continue uninterrupted in temporary locations during the renovation, past tense is used to refer to improvements made prior to the renovation, and future tense is used to refer to new Core services that will be provided after the renovation is completed in 2007. Therefore, the description of the Rodent Models Core reflects the strengths of past accomplishments as well as the promise of future achievements.