Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL) is a condition that affects the brain's small blood vessels and can lead to dementia (memory loss and other thinking problems). This study wants to learn more about what causes CADASIL and how it progresses. Researchers will follow 575 people for up to 5 years. Participants must be at least 18 years old and have a positive family history of CADASIL, shown by a genetic test called NOTCH3. They will need to visit the study center three times over three years for interviews, memory tests, an MRI scan (a picture of the brain), and blood tests. In between, there will be remote check-ins. Each participant needs a "study companion," someone who knows them well.

• Study lasts up to 5 years with 3 in-person visits and remote check-ins.
• Participants need a positive NOTCH3 test and a "study companion."
• Study includes memory tests, MRI scans, and blood draws.