This article discusses the need for biomarkers and surrogate endpoints for future clinical trials in individuals at risk for Huntington's disease. Definitions and criteria are presented for biomarkers and surrogate endpoints, and data are presented suggesting that striatal volumes, as measured on MRI scans, meet the criteria for a biomarker. Biomarkers can be used in lieu of clinical endpoints in treatment trials if there is evidence that treatment affects the biomarker in a way that is predictive of endpoint status. Because there are currently no effective treatments for Huntington's disease, it is not yet possible to validate whether change in MRI striatal volumes can serve as an effective surrogate endpoint. It is recommended that future clinical trials be designed using MRI striatal volumes to "screen" potential treatments. Those treatments that reduce the rate of striatal atrophy can then be tested with delay of symptom onset as the clinical endpoint. This strategy is essential if efficient and cost-effective clinical trials are to be conducted in the preclinical stage of Huntington's disease.